webview: Post web message listener to app looper

This was crashing with immediate_crash. Posting
the web message listener to the current looper thread
so that it rather crashes as a regular AndroidRuntime
exception.

Confirmed this worked by throwing a runtime
exception on my own custom WebViewCompat.WebMessageListener and
confirmed that the exception was being logged
out to AndroidRuntime instead of System.err from Chromium.

Bug: 1479041
Change-Id: I4edd0a55a5e752177b2cc71fcf2cc876cfc38ff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4847457
Commit-Queue: Rupert Wiser <bewise@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1193643}

android_webview/java/src/org/chromium/android_webview/WebMessageListenerHolder.java

@@ -21,7 +21,7 @@ import org.chromium.content_public.browser.MessagePort;
 @Lifetime.Temporary
 @JNINamespace("android_webview")
 public class WebMessageListenerHolder {
-    private WebMessageListener mListener;
+    private final WebMessageListener mListener;
 
     public WebMessageListenerHolder(@NonNull WebMessageListener listener) {
         mListener = listener;
@@ -30,7 +30,10 @@ public class WebMessageListenerHolder {
     @CalledByNative
     public void onPostMessage(MessagePayload payload, String sourceOrigin, boolean isMainFrame,
             MessagePort[] ports, JsReplyProxy replyProxy) {
-        mListener.onPostMessage(payload, Uri.parse(sourceOrigin), isMainFrame, replyProxy, ports);
+        AwThreadUtils.postToCurrentLooper(() -> {
+            mListener.onPostMessage(
+                    payload, Uri.parse(sourceOrigin), isMainFrame, replyProxy, ports);
+        });
     }
 
     public WebMessageListener getListener() {