Allow media track switching.

media/base/video_renderer.h

Index: media/base/video_renderer.h
diff --git a/media/base/video_renderer.h b/media/base/video_renderer.h
index 168b7e8ff0771e46178f37c47c553b6092f63616..7b6ba900a22ba11c3028dfba1fd3305db3de9ca6 100644
--- a/media/base/video_renderer.h
+++ b/media/base/video_renderer.h
@@ -33,6 +33,11 @@ class MEDIA_EXPORT VideoRenderer {
   //
   // |wall_clock_time_cb| is used to convert media timestamps into wallclock
   // timestamps.
+  //
+  // VideoRenderer may be reinitialized for playback of a different demuxer
+  // stream by calling Initialize again when the renderer is in a flushed
+  // state (i.e. after Flush call, but before StartPlayingFrom). This is used
+  // for media track switching.
   virtual void Initialize(DemuxerStream* stream,
                           CdmContext* cdm_context,
                           RendererClient* client,