Add a virtual destructor to WebNotificationDelegate.

public/platform/modules/notifications/WebNotificationDelegate.h

 // Copyright 2014 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 WebNotificationDelegate_h
 #define WebNotificationDelegate_h
 
 namespace blink {
 
 // A delegate through which the embedder can trigger events on a Document-bound
 // Web Notifications object. Service Worker-bound Web Notifications will not have
 // a delegate, as their events will be fired on a Service Worker instead.
 class WebNotificationDelegate {
 public:
+    virtual ~WebNotificationDelegate() {}
+
     virtual void dispatchClickEvent() = 0;
     virtual void dispatchShowEvent() = 0;
     virtual void dispatchErrorEvent() = 0;
     virtual void dispatchCloseEvent() = 0;
 };
 
 } // namespace blink
 
 #endif // WebNotificationDelegate_h