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

content/renderer/media/webmediaplayer_ms_unittest.cc

Index: content/renderer/media/webmediaplayer_ms_unittest.cc
diff --git a/content/renderer/media/webmediaplayer_ms_unittest.cc b/content/renderer/media/webmediaplayer_ms_unittest.cc
index 8edea5674922259d11826c7a9ef23d83dd4dfaa7..5d2e9990c9af9e80157f1308328fb4d2b0e4e3d7 100644
--- a/content/renderer/media/webmediaplayer_ms_unittest.cc
+++ b/content/renderer/media/webmediaplayer_ms_unittest.cc
@@ -94,6 +94,10 @@ class FakeWebMediaPlayerDelegate
     return false;
   }
 
+  void SetIsFullscreen(int delegate_id, bool is_fullscreen) override {
+    EXPECT_EQ(delegate_id_, delegate_id);
+  }
+
   bool IsFrameHidden() override { return is_hidden_; }
   bool IsFrameClosed() override { return false; }
   bool IsBackgroundVideoPlaybackUnlocked() override { return false; }