MutationObserver: Fix a null-pointer dereference in MutationObserverRegistration::unregister.

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

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 18 matching lines @@
  */
 
 #ifndef MutationCallback_h
 #define MutationCallback_h
 
 #include "platform/heap/Handle.h"
 #include "wtf/Vector.h"
 
 namespace blink {
 
+class ExecutionContext;
 class MutationRecord;
 class MutationObserver;
 
 class MutationCallback : public GarbageCollectedFinalized<MutationCallback> {
  public:
   virtual ~MutationCallback() {}
 
   virtual void call(const HeapVector<Member<MutationRecord>>&,
                     MutationObserver*) = 0;
   virtual ExecutionContext* getExecutionContext() const = 0;
 
   DEFINE_INLINE_VIRTUAL_TRACE() {}
 };
 
 }  // namespace blink
 
 #endif  // MutationCallback_h