Android: Add @MonotonicNonNull and use it in a few spots

Support for this annotation was recently added to NullAway. It removes
some uses of assumeNonNull().

Note: It does not yet work with static fields. Reported here:
https://github.com/uber/NullAway/issues/1148#issuecomment-2707363431

Bug: 389129271
Change-Id: I1c24769b48cb381ba6a5974b24d04f2c772d226d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6334458
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1429766}

build/android/BUILD.gn

@@ -56,6 +56,7 @@ if (enable_java_templates) {
       "java/src/org/chromium/build/annotations/IdentifierNameString.java",
       "java/src/org/chromium/build/annotations/Initializer.java",
       "java/src/org/chromium/build/annotations/MockedInTests.java",
+      "java/src/org/chromium/build/annotations/MonotonicNonNull.java",
       "java/src/org/chromium/build/annotations/NullMarked.java",
       "java/src/org/chromium/build/annotations/NullUnmarked.java",
       "java/src/org/chromium/build/annotations/Nullable.java",

build/android/java/src/org/chromium/build/annotations/MonotonicNonNull.java

@@ -0,0 +1,20 @@
+// Copyright 2025 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.build.annotations;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Fields that are never null once becoming non-null. Use to avoid warnings when such fields are
+ * accessed by lambdas after being assigned.
+ *
+ * <p>See: https://github.com/uber/NullAway/wiki/Supported-Annotations
+ */
+@Target(ElementType.FIELD)
+@Retention(RetentionPolicy.CLASS)
+public @interface MonotonicNonNull {}

components/browser_ui/site_settings/android/java/src/org/chromium/components/browser_ui/site_settings/ClearWebsiteStorageDialog.java

@@ -15,6 +15,7 @@ import androidx.preference.Preference;
 import androidx.preference.PreferenceDialogFragmentCompat;
 
 import org.chromium.base.Callback;
+import org.chromium.build.annotations.MonotonicNonNull;
 import org.chromium.build.annotations.NullMarked;
 import org.chromium.build.annotations.Nullable;
 import org.chromium.ui.base.ViewUtils;
@@ -28,7 +29,7 @@ public class ClearWebsiteStorageDialog extends PreferenceDialogFragmentCompat {
     private static @Nullable Callback<Boolean> sCallback;
 
     // The view containing the dialog ui elements.
-    private @Nullable View mDialogView;
+    private @MonotonicNonNull View mDialogView;
 
     public static ClearWebsiteStorageDialog newInstance(
             Preference preference, Callback<Boolean> callback, boolean isGroup) {
@@ -65,7 +66,7 @@ public class ClearWebsiteStorageDialog extends PreferenceDialogFragmentCompat {
                     .post(
                             () -> {
                                 ViewUtils.requestLayout(
-                                        assumeNonNull(mDialogView),
+                                        mDialogView,
                                         "ClearWebsiteStorageDialog.onConfigurationChanged"
                                                 + " Runnable");
                             });

components/messages/android/java/src/org/chromium/components/messages/MessageWrapper.java

@@ -4,8 +4,6 @@
 
 package org.chromium.components.messages;
 
-import static org.chromium.build.NullUtil.assumeNonNull;
-
 import android.content.Context;
 import android.graphics.Bitmap;
 import android.graphics.drawable.BitmapDrawable;
@@ -18,6 +16,7 @@ import org.jni_zero.CalledByNative;
 import org.jni_zero.JNINamespace;
 import org.jni_zero.NativeMethods;
 
+import org.chromium.build.annotations.MonotonicNonNull;
 import org.chromium.build.annotations.NullMarked;
 import org.chromium.build.annotations.Nullable;
 import org.chromium.ui.base.WindowAndroid;
@@ -31,10 +30,11 @@ import org.chromium.ui.modelutil.PropertyModel;
 public final class MessageWrapper implements ListMenu.Delegate {
     private long mNativeMessageWrapper;
     private final PropertyModel mMessageProperties;
-    private @Nullable MessageSecondaryMenuItems mMessageSecondaryMenuItems;
+    private @MonotonicNonNull MessageSecondaryMenuItems mMessageSecondaryMenuItems;
 
     /**
      * Creates an instance of MessageWrapper and links it with native MessageWrapper object.
+     *
      * @param nativeMessageWrapper Pointer to native MessageWrapper.
      * @param messageIdentifier Message identifier of the new message.
      * @return reference to created MessageWrapper.
@@ -137,7 +137,7 @@ public final class MessageWrapper implements ListMenu.Delegate {
             mMessageProperties.set(MessageBannerProperties.SECONDARY_MENU_MAX_SIZE, maxSize);
             mMessageProperties.set(
                     MessageBannerProperties.SECONDARY_MENU_BUTTON_DELEGATE,
-                    () -> assumeNonNull(mMessageSecondaryMenuItems).createListMenu(context, this));
+                    () -> mMessageSecondaryMenuItems.createListMenu(context, this));
         }
     }
 

styleguide/java/nullaway.md

@@ -116,6 +116,22 @@ private boolean isParamNonNull(@Nullable String foo) {
 }
 ```
 
+### Field Annotations
+
+```java
+// Starts as null, but may not be assigned a nullable value.
+private @MonotonicNonNull String mSomeValue;
+
+public void doThing(String value) {
+    // Emits a warning since mSomeValue is nullable:
+    helper(mSomeValue);
+
+    mSomeValue = value;
+    // No warning about mSomeValue being nullable, even though it's used in a lambda.
+    PostTask.postTask(TaskTraits.USER_BLOCKING, () -> helper(mSomeValue));
+}
+```
+
 ### "assert", "assumeNonNull()", and "requireNonNull()"
 
 ```java