Fix crash in MutationObserver::canDeliver due to NULL ExecutionContext

Source/bindings/core/v8/V8MutationCallback.h

 /*
  * Copyright (C) 2013 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 29 matching lines @@
 
 class V8MutationCallback final : public MutationCallback, public ActiveDOMCallback {
 public:
     static PassOwnPtr<V8MutationCallback> create(v8::Handle<v8::Function> callback, v8::Handle<v8::Object> owner, ScriptState* scriptState)
     {
         return adoptPtr(new V8MutationCallback(callback, owner, scriptState));
     }
 
     virtual void call(const WillBeHeapVector<RefPtrWillBeMember<MutationRecord> >&, MutationObserver*) override;
     virtual ExecutionContext* executionContext() const override { return ContextLifecycleObserver::executionContext(); }
+    virtual bool canInvokeCallback() const override { return ActiveDOMCallback::canInvokeCallback(); }
 
 private:
     V8MutationCallback(v8::Handle<v8::Function>, v8::Handle<v8::Object>, ScriptState*);
 
     static void setWeakCallback(const v8::WeakCallbackData<v8::Function, V8MutationCallback>&);
 
     ScopedPersistent<v8::Function> m_callback;
     RefPtr<ScriptState> m_scriptState;
 };
 
 }
 
 #endif // V8MutationCallback_h