Remove PageVisibilityObserver contextDestroyed() notifications.

third_party/WebKit/Source/core/dom/SynchronousMutationObserver.h

 // Copyright 2016 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 SynchronousMutationObserver_h
 #define SynchronousMutationObserver_h
 
 #include "base/macros.h"
 #include "core/CoreExport.h"
 #include "platform/LifecycleObserver.h"
@@ skipping 47 matching lines @@
                                       unsigned offset,
                                       unsigned oldLength,
                                       unsigned newLength);
 
   // Called before removing container node.
   virtual void nodeChildrenWillBeRemoved(ContainerNode&);
 
   // Called before removing node.
   virtual void nodeWillBeRemoved(Node&);
 
+  // Called when detaching document.
+  virtual void contextDestroyed(Document*) {}
+
  protected:
   SynchronousMutationObserver();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SynchronousMutationObserver);
 };
 
 }  // namespace blink
 
 #endif  // SynchronousMutationObserver_h