Notify WebMediaPlayer when its ancestor enters/exists fullscreen.

third_party/WebKit/Source/core/dom/Fullscreen.h

Index: third_party/WebKit/Source/core/dom/Fullscreen.h
diff --git a/third_party/WebKit/Source/core/dom/Fullscreen.h b/third_party/WebKit/Source/core/dom/Fullscreen.h
index 34b28d69d10eab6b61f507048405d53a1f6ef7c8..ea75c8385dcf4cb1b105f791d9db698fb150b820 100644
--- a/third_party/WebKit/Source/core/dom/Fullscreen.h
+++ b/third_party/WebKit/Source/core/dom/Fullscreen.h
@@ -41,6 +41,8 @@
 #include "wtf/RefPtr.h"
 #include "wtf/Vector.h"
 
+#include <set>
+
 namespace blink {
 
 class LayoutFullScreen;
@@ -61,6 +63,14 @@ class CORE_EXPORT Fullscreen final
   static Element* currentFullScreenElementFrom(Document&);
   static bool isCurrentFullScreenElement(const Element&);
 
+  class FullscreenObserver {
+   public:
+    FullscreenObserver() {}
+    virtual ~FullscreenObserver() {}
+    virtual void onEnteredFullscreen() = 0;
+    virtual void onExitedFullscreen() = 0;
+  };
+
   enum RequestType {
     // Element.requestFullscreen()
     UnprefixedRequest,
@@ -118,6 +128,9 @@ class CORE_EXPORT Fullscreen final
   // ContextLifecycleObserver:
   void contextDestroyed() override;
 
+  void registerObserver(FullscreenObserver*);
+  void removeObserver(FullscreenObserver*);
+
   DECLARE_VIRTUAL_TRACE();
 
  private:
@@ -153,6 +166,8 @@ class CORE_EXPORT Fullscreen final
   // postMessage can carry user gestures.  If that happens, this should be
   // moved to be part of |m_fullscreenElementStack|.
   bool m_forCrossProcessDescendant;
+
+  std::set<FullscreenObserver*> m_observers;
 };
 
 inline Fullscreen* Fullscreen::fromIfExists(Document& document) {