Detect change on 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..59d3bb118c022602af99633c72a7da82088ee1ea 100644
--- a/media/base/media_observer.h
+++ b/media/base/media_observer.h
@@ -20,6 +20,11 @@ class MEDIA_EXPORT MediaObserver {
   virtual void OnEnteredFullscreen() = 0;
   virtual void OnExitedFullscreen() = 0;
 
+  // Called when the media element starts/stops stably filling most of the
+  // viewport.
+  virtual void OnStablyFilledMostViewportChanged(

[ojan, 2016/11/24 00:48:33]

Nit: These methods would be easier to understand if you
s/MostViewport/Viewport/. The argument name makes clear that it's mostly
filling.

Feel free to ignore since this is just bikeshedding.

[xjz, 2016/11/24 02:16:16]

Done.

+      bool is_mostly_filling_viewport) {}
+
   // 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;