Detect change on the intersection of video and viewport.

third_party/WebKit/public/platform/WebMediaPlayer.h

Index: third_party/WebKit/public/platform/WebMediaPlayer.h
diff --git a/third_party/WebKit/public/platform/WebMediaPlayer.h b/third_party/WebKit/public/platform/WebMediaPlayer.h
index 75db1339d060c067c15e633cd2baa69f5af61843..2b01f6a5964df3ecd440449f0c806b0988b81079 100644
--- a/third_party/WebKit/public/platform/WebMediaPlayer.h
+++ b/third_party/WebKit/public/platform/WebMediaPlayer.h
@@ -228,6 +228,10 @@ class WebMediaPlayer {
   virtual void enteredFullscreen() {}
   virtual void exitedFullscreen() {}
 
+  // Inform WebmediaPlayer when the element starts/stops stably filling most of

[xhwang, 2016/11/30 18:13:09]

nit: WebMediaPlayer

[xjz, 2016/11/30 18:58:16]

Done.

+  // the viewport.
+  virtual void stablyFilledViewport(bool isMostlyFillingViewport) {}

[xhwang, 2016/11/30 18:13:09]

nit: IMHO this is a bit hard to read, especially since "stably" is in the API
name, but "mostly" is in the boolean parameter. For example, what's the
difference b/w:

stablyFilledViewport(bool isMostlyFillingViewport)

and 

mostlyFilledViewport(bool isStablyFillingViewport)

With that, how about something like:

stablyAndMostlyFilledViewport(bool filled)

or anything that could make the interface cleaner?

[xhwang, 2016/11/30 18:13:09]

nit: Could you please clarify what "stably" and "mostly" exactly mean?

[Z_DONOTUSE, 2016/11/30 18:48:23]

How about just filledViewport(bool isMostlyFillingViewport)? As we tweak
heuristics, the "stably" bit might change or get dropped entirely. We might also
add new things. 

[xjz, 2016/11/30 18:58:16]

I am fine with either way, but think this one is simpler. 
xhwang: WDYT?

[xhwang, 2016/11/30 19:06:21]

filledViewport(bool isMostlyFillingViewport) l-g-t-m :)

[miu, 2016/11/30 19:56:56]

I don't like filledViewport, for multiple reasons:

1. "filled" can read as an adjective, not a verb. That suggests a naming like
"didFillViewport" but that brings me to reason #2:

2. The naming of this function is exposing the internal details of the
heuristics, but we already know these details will be changing somewhat in a
soon-upcoming CL. So, the "filledViewport" naming locks in the current
implementation details, rather than exposing a callback interface for a specific
purpose.

So, let's step back and think about what this is being used for and what
information it is meant to communicate: that the video has been positioned and
sized in such a way to indicate the user is meant to view or engage with it
immersively. Therefore, I propose:

   onShowingImmersively(bool)         <-- my favorite
or becameDominantVisibleContent(bool)
or becamePrimaryEngagingElement(bool)

Which one do you guys like?

+
   virtual void enabledAudioTracksChanged(
       const WebVector<TrackId>& enabledTrackIds) {}
   // |selectedTrackId| is null if no track is selected.