[android_webview] Factor AwWebContentsObserver out of AwContentsClient.

android_webview/java/src/org/chromium/android_webview/AwContents.java

Index: android_webview/java/src/org/chromium/android_webview/AwContents.java
diff --git a/android_webview/java/src/org/chromium/android_webview/AwContents.java b/android_webview/java/src/org/chromium/android_webview/AwContents.java
index fc456df4a0d89ff83c3ce0ec1e57b8d2fa341ba2..8338559035220c8762571e0d4a9627c13eeb1464 100644
--- a/android_webview/java/src/org/chromium/android_webview/AwContents.java
+++ b/android_webview/java/src/org/chromium/android_webview/AwContents.java
@@ -52,6 +52,7 @@ import org.chromium.content.browser.ContentViewStatics;
 import org.chromium.content.browser.LoadUrlParams;
 import org.chromium.content.browser.NavigationHistory;
 import org.chromium.content.browser.PageTransitionTypes;
+import org.chromium.content.browser.WebContentsObserverAndroid;
 import org.chromium.content.common.CleanupReference;
 import org.chromium.content_public.Referrer;
 import org.chromium.content_public.browser.GestureStateListener;
@@ -182,6 +183,7 @@ public class AwContents {
     private ContentViewCore mContentViewCore;
     private final AwContentsClient mContentsClient;
     private final AwContentViewClient mContentViewClient;
+    private WebContentsObserverAndroid mWebContentsObserver;
     private final AwContentsClientBridge mContentsClientBridge;
     private final AwWebContentsDelegateAdapter mWebContentsDelegate;
     private final AwContentsIoThreadClient mIoThreadClient;
@@ -758,7 +760,7 @@ public class AwContents {
                 new AwGestureStateListener(), mContentViewClient, mZoomControls);
         nativeSetJavaPeers(mNativeAwContents, this, mWebContentsDelegate, mContentsClientBridge,
                 mIoThreadClient, mInterceptNavigationDelegate);
-        mContentsClient.installWebContentsObserver(mContentViewCore.getWebContents());
+        installWebContentsObserver();
         mSettings.setWebContents(nativeWebContents);
         nativeSetDipScale(mNativeAwContents, (float) mDIPScale);
 
@@ -766,6 +768,14 @@ public class AwContents {
         mContentViewCore.onShow();
     }
 
+    private void installWebContentsObserver() {
+        if (mWebContentsObserver != null) {
+            mWebContentsObserver.detachFromWebContents();
+        }
+        mWebContentsObserver = new AwWebContentsObserver(mContentViewCore.getWebContents(),
+                mContentsClient);
+    }
+
     /**
      * Called on the "source" AwContents that is opening the popup window to
      * provide the AwContents to host the pop up content.