JNI Zero: Require @JniType conversions functions to appear in .h files

This has the following advantages:
 * Missing conversions will be a compile-time error rather than
   link-time.
 * Allows us to define a default implementations that maps a pointer
   type to its dereferenced type.
 * Allows us to define a default implementations for std::optional<T>.

Bug: 334905258
Change-Id: I092261e163c88c4e3a235e7f74059c5630cb5ea2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5559723
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1305161}

components/prefs/android/pref_service_android.cc

@@ -6,25 +6,17 @@
 
 #include <string>
 
-#include "components/prefs/android/jni_headers/PrefService_jni.h"
+#include "base/android/jni_string.h"
 #include "components/prefs/pref_service.h"
 #include "components/prefs/prefs_export.h"
 
-using base::android::JavaParamRef;
-using base::android::JavaRef;
-using base::android::ScopedJavaLocalRef;
+// Must come after all headers that specialize FromJniType() / ToJniType().
+#include "components/prefs/android/jni_headers/PrefService_jni.h"
+
 using jni_zero::AttachCurrentThread;
-
-namespace jni_zero {
-
-template <>
-COMPONENTS_PREFS_EXPORT PrefService* FromJniType<PrefService*>(
-    JNIEnv* env,
-    const JavaRef<jobject>& obj) {
-  return PrefServiceAndroid::FromPrefServiceAndroid(obj);
-}
-
-}  // namespace jni_zero
+using jni_zero::JavaParamRef;
+using jni_zero::JavaRef;
+using jni_zero::ScopedJavaLocalRef;
 
 PrefServiceAndroid::PrefServiceAndroid(PrefService* pref_service)
     : pref_service_(pref_service) {}

components/prefs/android/pref_service_android.h

@@ -5,10 +5,7 @@
 #ifndef COMPONENTS_PREFS_ANDROID_PREF_SERVICE_ANDROID_H_
 #define COMPONENTS_PREFS_ANDROID_PREF_SERVICE_ANDROID_H_
 
-#include <jni.h>
-
-#include "base/android/jni_string.h"
-#include "base/android/scoped_java_ref.h"
+#include "base/android/jni_android.h"
 #include "base/memory/raw_ptr.h"
 #include "components/prefs/prefs_export.h"
 
@@ -67,4 +64,12 @@ class COMPONENTS_PREFS_EXPORT PrefServiceAndroid {
   base::android::ScopedJavaGlobalRef<jobject> java_ref_;
 };
 
+namespace jni_zero {
+template <>
+inline PrefService* FromJniType<PrefService*>(JNIEnv* env,
+                                              const JavaRef<jobject>& obj) {
+  return PrefServiceAndroid::FromPrefServiceAndroid(obj);
+}
+}  // namespace jni_zero
+
 #endif  // COMPONENTS_PREFS_ANDROID_PREF_SERVICE_ANDROID_H_