Detect change on the intersection of video and viewport.

media/base/media_observer.h

 // Copyright 2016 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 MEDIA_BASE_MEDIA_OBSERVER_H_
 #define MEDIA_BASE_MEDIA_OBSERVER_H_
 
 #include "media/base/cdm_context.h"
 #include "media/base/pipeline_metadata.h"
 
 namespace media {
 
 // This class is an observer of media player events.
 class MEDIA_EXPORT MediaObserver {
  public:
   MediaObserver();
   virtual ~MediaObserver();
 
   // Called when the media element entered/exited fullscreen.
   virtual void OnEnteredFullscreen() = 0;
   virtual void OnExitedFullscreen() = 0;
 
+  // Called when the media element starts/stops being the dominant visible
+  // content.
+  virtual void OnBecameDominantVisibleContent(bool is_dominant) {}
+
   // Called when CDM is attached to the media element. The |cdm_context| is
   // only guaranteed to be valid in this call.
   virtual void OnSetCdm(CdmContext* cdm_context) = 0;
 
   // Called after demuxer is initialized.
   virtual void OnMetadataChanged(const PipelineMetadata& metadata) = 0;
 
   // Called to indicate whether the site requests that remote playback be
   // disabled. The "disabled" naming corresponds with the
   // "disableRemotePlayback" media element attribute, as described in the
   // Remote Playback API spec: https://w3c.github.io/remote-playback
   virtual void OnRemotePlaybackDisabled(bool disabled) = 0;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_MEDIA_OBSERVER_H_