Android WebView: clean up the AwContentsClientBridge webcontents userdata.

android_webview/native/aw_contents_client_bridge.cc

Index: android_webview/native/aw_contents_client_bridge.cc
diff --git a/android_webview/native/aw_contents_client_bridge.cc b/android_webview/native/aw_contents_client_bridge.cc
index 5431be749a6302a14e7762d752257d4e648ba6c7..6596b72a69b86ae0e1274b771a0cb37dc7072a27 100644
--- a/android_webview/native/aw_contents_client_bridge.cc
+++ b/android_webview/native/aw_contents_client_bridge.cc
@@ -262,8 +262,10 @@ void AwContentsClientBridge::RunJavaScriptDialog(
   JNIEnv* env = AttachCurrentThread();
 
   ScopedJavaLocalRef<jobject> obj = java_ref_.get(env);
-  if (obj.is_null())
+  if (obj.is_null()) {
+    callback.Run(false, base::string16());
     return;
+  }
 
   int callback_id = pending_js_dialog_callbacks_.Add(
       new content::JavaScriptDialogManager::DialogClosedCallback(callback));