Dispose of DOMTimer upon explicit ExecutionContext unregistration.

Source/core/frame/DOMTimer.h

 /*
  * Copyright (C) 2008 Apple 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 40 matching lines @@
     // ActiveDOMObject
     virtual void contextDestroyed() override;
     virtual void stop() override;
 
     // The following are essentially constants. All intervals are in seconds.
     static double hiddenPageAlignmentInterval();
     static double visiblePageAlignmentInterval();
 
     void trace(Visitor*) { }
 
+    void dispose();
+
 private:
     friend class ExecutionContext; // For create().
 
     // Should only be used by ExecutionContext.
     static PassOwnPtrWillBeRawPtr<DOMTimer> create(ExecutionContext* context, PassOwnPtr<ScheduledAction> action, int timeout, bool singleShot, int timeoutID)
     {
         return adoptPtrWillBeNoop(new DOMTimer(context, action, timeout, singleShot, timeoutID));
     }
 
     DOMTimer(ExecutionContext*, PassOwnPtr<ScheduledAction>, int interval, bool singleShot, int timeoutID);
     virtual void fired() override;
 
     // Retuns timer fire time rounded to the next multiple of timer alignment interval.
     virtual double alignedFireTime(double) const override;
 
     int m_timeoutID;
     int m_nestingLevel;
     OwnPtr<ScheduledAction> m_action;
     RefPtr<UserGestureToken> m_userGestureToken;
 };
 
 } // namespace blink
 
 #endif // DOMTimer_h