Fixing a crash in MediaCustomControlsFullscreenDetector when the page is destroyed

third_party/WebKit/Source/core/html/MediaCustomControlsFullscreenDetector.h

 // Copyright 2017 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 MediaCustomControlsFullscreenDetector_h
 #define MediaCustomControlsFullscreenDetector_h
 
 #include "core/CoreExport.h"
+#include "core/dom/ContextLifecycleObserver.h"
 #include "core/events/EventListener.h"
 #include "platform/Timer.h"
 
 namespace blink {
 
 class HTMLVideoElement;
 class IntRect;
 class TimerBase;
 
 class CORE_EXPORT MediaCustomControlsFullscreenDetector final
-    : public EventListener {
+    : public EventListener,
+      public ContextLifecycleObserver {
+  USING_GARBAGE_COLLECTED_MIXIN(MediaCustomControlsFullscreenDetector);
   WTF_MAKE_NONCOPYABLE(MediaCustomControlsFullscreenDetector);
 
  public:
   explicit MediaCustomControlsFullscreenDetector(HTMLVideoElement&);
 
   // EventListener implementation.
   bool operator==(const EventListener&) const override;
 
+  // ContextLifecycleObserver implemnetation.
+  void contextDestroyed(ExecutionContext*);
+
   DECLARE_VIRTUAL_TRACE();
 
  private:
   friend class MediaCustomControlsFullscreenDetectorTest;
+  friend class HTMLMediaElementEventListenersTest;
 
   // EventListener implementation.
   void handleEvent(ExecutionContext*, Event*) override;
 
   void attach();
   void detach();
 
   HTMLVideoElement& videoElement() { return *m_videoElement; }
 
   void onCheckViewportIntersectionTimerFired(TimerBase*);
 
   bool isVideoOrParentFullscreen();
 
   static bool computeIsDominantVideoForTests(const IntRect& targetRect,
                                              const IntRect& rootRect,
                                              const IntRect& intersectionRect);
 
   // `m_videoElement` owns |this|.
   Member<HTMLVideoElement> m_videoElement;
   TaskRunnerTimer<MediaCustomControlsFullscreenDetector>
       m_checkViewportIntersectionTimer;
 };
 
 }  // namespace blink
 
 #endif  // MediaCustomControlsFullscreenDetector_h