Implement Permission change observing in Blink.

public/platform/modules/permissions/WebPermissionClient.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef WebPermissionClient_h
 #define WebPermissionClient_h
 
 #include "public/platform/WebCallbacks.h"
 #include "public/platform/modules/permissions/WebPermissionStatus.h"
 #include "public/platform/modules/permissions/WebPermissionType.h"
 
 namespace blink {
 
 using WebPermissionQueryCallback = WebCallbacks<WebPermissionStatus, void>;
 
+class WebPermissionObserver;
 class WebURL;
 
 // This client is expected to do general permission handling. From simple
 // permission query to requesting new permissions.
 class WebPermissionClient {
 public:
     // Query the permission status of a given origin for a specific permission.
-    virtual void queryPermission(WebPermissionType, const WebURL&, WebPermissionQueryCallback*) = 0;
+    virtual void queryPermission(WebPermissionType, const WebURL&, WebPermissionQueryCallback*) { }
+
+    // Listen for permission changes for a given origin and inform the observer
+    // when they happen. The observer is not owned by the WebPermissionClient.
+    // The client should expect stopListening() to be called when the observer
+    // will be destroyed.
+    virtual void startListening(WebPermissionType, const WebURL&, WebPermissionObserver*) { }
+
+    // Stop listening to all the permission changes associated with the given
+    // observer.
+    virtual void stopListening(WebPermissionObserver*) { }
 
 protected:
     virtual ~WebPermissionClient() { }
 };
 
 } // namespace blink
 
 #endif // WebPermissionClient_h