Disable background video track behind a feature flag

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

Index: third_party/WebKit/Source/core/html/HTMLMediaElement.h
diff --git a/third_party/WebKit/Source/core/html/HTMLMediaElement.h b/third_party/WebKit/Source/core/html/HTMLMediaElement.h
index 7e93de204668e5ba00636fb503c02f15fae9a430..b3320956acfa628bbb9de91fb339753d6d27e8bb 100644
--- a/third_party/WebKit/Source/core/html/HTMLMediaElement.h
+++ b/third_party/WebKit/Source/core/html/HTMLMediaElement.h
@@ -391,6 +391,8 @@ class CORE_EXPORT HTMLMediaElement : public HTMLElement,
   void disconnectedFromRemoteDevice() final;
   void cancelledRemotePlaybackRequest() final;
   void remotePlaybackStarted() final;
+  void hidden() final;
+  void shown() final;
   bool isAutoplayingMuted() final;
   void requestReload(const WebURL&) final;
 
@@ -720,6 +722,7 @@ class CORE_EXPORT HTMLMediaElement : public HTMLElement,
   friend class TrackDisplayUpdateScope;
   friend class AutoplayExperimentHelper;
   friend class MediaControlsTest;
+  friend class HTMLVideoElementTest;
 
   Member<AutoplayExperimentHelper::Client> m_autoplayHelperClient;
   Member<AutoplayExperimentHelper> m_autoplayHelper;