Provide a WebContents API to discover the playback of a fullscreen video.

media/blink/webmediaplayer_impl_unittest.cc

Index: media/blink/webmediaplayer_impl_unittest.cc
diff --git a/media/blink/webmediaplayer_impl_unittest.cc b/media/blink/webmediaplayer_impl_unittest.cc
index e66351be6ca3629b9923dc00bd4da347dd150d4d..78bebf32ad67dfd97682972ea9bbb2d983159d26 100644
--- a/media/blink/webmediaplayer_impl_unittest.cc
+++ b/media/blink/webmediaplayer_impl_unittest.cc
@@ -146,6 +146,10 @@ class MockWebMediaPlayerDelegate : public WebMediaPlayerDelegate {
     return is_stale_;
   }
 
+  void SetIsEffectivelyFullscreen(int player_id, bool value) override {
+    DCHECK_EQ(player_id_, player_id);
+  }
+
   bool IsFrameHidden() override { return is_hidden_; }
 
   bool IsFrameClosed() override { return is_closed_; }