[aw] Unregister delegates from AwContents.destroy()

AwContents are deleted asynchronously, so to stop receiving
callbacks we unregister delegates / observers / listeners
from AwContents.destroy() and ContentViewCore.destroy().
Receiving callbacks after AwContents.destroy() is called is
problematic because Java refs have already been set to null.

BUG=415666

Review URL: https://codereview.chromium.org/594103002

Cr-Commit-Position: refs/heads/master@{#296977}

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

@@ -886,6 +886,8 @@ public class AwContents {
                 nativeOnDetachedFromWindow(mNativeAwContents);
             }
 
+            mWebContentsObserver.detachFromWebContents();
+            mWebContentsObserver = null;
             mContentViewCore.destroy();
             mContentViewCore = null;
             mNativeAwContents = 0;

android_webview/native/aw_contents.cc

@@ -300,6 +300,10 @@ void AwContents::Destroy(JNIEnv* env, jobject obj) {
   AwContentsClientBridgeBase::Disassociate(web_contents_.get());
   contents_client_bridge_.reset();
 
+  // Do not wait until the WebContents are deleted asynchronously to clear
+  // the delegate and stop sending callbacks.
+  web_contents_->SetDelegate(NULL);
+
   // We do not delete AwContents immediately. Some applications try to delete
   // Webview in ShouldOverrideUrlLoading callback, which is a sync IPC from
   // Webkit.

content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java

@@ -110,6 +110,16 @@ public class ContentViewCore
     private static final int IS_LONG_PRESS = 1;
     private static final int IS_LONG_TAP = 2;
 
+    private static final ZoomControlsDelegate NO_OP_ZOOM_CONTROLS_DELEGATE =
+            new ZoomControlsDelegate() {
+        @Override
+        public void invokeZoomPicker() {}
+        @Override
+        public void dismissZoomPicker() {}
+        @Override
+        public void updateZoomControls() {}
+    };
+
     // If the embedder adds a JavaScript interface object that contains an indirect reference to
     // the ContentViewCore, then storing a strong ref to the interface object on the native
     // side would prevent garbage collection of the ContentViewCore (as that strong ref would
@@ -603,14 +613,7 @@ public class ContentViewCore
         long viewAndroidNativePointer = mViewAndroid.getNativePointer();
         assert viewAndroidNativePointer != 0;
 
-        mZoomControlsDelegate = new ZoomControlsDelegate() {
-            @Override
-            public void invokeZoomPicker() {}
-            @Override
-            public void dismissZoomPicker() {}
-            @Override
-            public void updateZoomControls() {}
-        };
+        mZoomControlsDelegate = NO_OP_ZOOM_CONTROLS_DELEGATE;
 
         mNativeContentViewCore = nativeInit(
                 nativeWebContents, viewAndroidNativePointer, windowNativePointer,
@@ -779,6 +782,16 @@ public class ContentViewCore
         if (mNativeContentViewCore != 0) {
             nativeOnJavaContentViewCoreDestroyed(mNativeContentViewCore);
         }
+        mWebContentsObserver.detachFromWebContents();
+        mWebContentsObserver = null;
+        setSmartClipDataListener(null);
+        setZoomControlsDelegate(null);
+        // TODO(igsolla): address TODO in ContentViewClient because ContentViewClient is not
+        // currently a real Null Object.
+        //
+        // Instead of deleting the client we use the Null Object pattern to avoid null checks
+        // in this class.
+        mContentViewClient = new ContentViewClient();
         mWebContents = null;
         if (mViewAndroid != null) mViewAndroid.destroy();
         mNativeContentViewCore = 0;
@@ -1758,6 +1771,10 @@ public class ContentViewCore
     }
 
     public void setZoomControlsDelegate(ZoomControlsDelegate zoomControlsDelegate) {
+        if (zoomControlsDelegate == null) {
+            mZoomControlsDelegate = NO_OP_ZOOM_CONTROLS_DELEGATE;
+            return;
+        }
         mZoomControlsDelegate = zoomControlsDelegate;
     }