Eagerly remove disposed DOMTimers as observers.

third_party/WebKit/Source/core/frame/DOMTimer.cpp

 /*
  * 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 78 matching lines @@
 
 DOMTimer::~DOMTimer()
 {
 }
 
 void DOMTimer::disposeTimer()
 {
     m_action = nullptr;
     m_userGestureToken = nullptr;
     stop();
+    clearContext();
 }
 
 void DOMTimer::fired()
 {
     ExecutionContext* context = getExecutionContext();
     ASSERT(context);
     context->timers()->setTimerNestingLevel(m_nestingLevel);
     ASSERT(!context->activeDOMObjectsAreSuspended());
     // Only the first execution of a multi-shot timer should get an affirmative user gesture indicator.
     UserGestureIndicator gestureIndicator(m_userGestureToken.release());
@@ skipping 44 matching lines @@
     return getExecutionContext()->timers()->timerTaskRunner();
 }
 
 DEFINE_TRACE(DOMTimer)
 {
     visitor->trace(m_action);
     SuspendableTimer::trace(visitor);
 }
 
 } // namespace blink