Monitor the intersection of video and viewport.

media/base/media_observer.h

Index: media/base/media_observer.h
diff --git a/media/base/media_observer.h b/media/base/media_observer.h
index c3e29a1789f173d2cc62ad12845392cfaf1a70f7..510c38e63e90e7aa0a03f482a5bd2bf12814007b 100644
--- a/media/base/media_observer.h
+++ b/media/base/media_observer.h
@@ -19,6 +19,8 @@ class MEDIA_EXPORT MediaObserver {
   // Called when the media element entered/exited fullscreen.
   virtual void OnEnteredFullscreen() = 0;
   virtual void OnExitedFullscreen() = 0;
+  // Called when the video/viewport changed.
+  virtual void OnVideoViewportRatioChanged(double ratio) {}
 
   // Called when CDM is attached to the media element. The |cdm_context| is
   // only guaranteed to be valid in this call.