Fixing a crash in MediaCustomControlsFullscreenDetector when the page is destroyed

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

Index: third_party/WebKit/Source/core/html/MediaCustomControlsFullscreenDetector.h
diff --git a/third_party/WebKit/Source/core/html/MediaCustomControlsFullscreenDetector.h b/third_party/WebKit/Source/core/html/MediaCustomControlsFullscreenDetector.h
index bf976182e8cbc5be37a8bcac1101f2c49640f576..4d0f2154e7eaba969c94c05f9ac69ed1750d6ff6 100644
--- a/third_party/WebKit/Source/core/html/MediaCustomControlsFullscreenDetector.h
+++ b/third_party/WebKit/Source/core/html/MediaCustomControlsFullscreenDetector.h
@@ -6,6 +6,7 @@
 #define MediaCustomControlsFullscreenDetector_h
 
 #include "core/CoreExport.h"
+#include "core/dom/ContextLifecycleObserver.h"
 #include "core/events/EventListener.h"
 #include "platform/Timer.h"
 
@@ -16,7 +17,9 @@ 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:
@@ -25,10 +28,14 @@ class CORE_EXPORT MediaCustomControlsFullscreenDetector final
   // 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;