Add support for external video renderers in mojo.

media/mojo/interfaces/media_renderer.mojom

Index: media/mojo/interfaces/media_renderer.mojom
diff --git a/media/mojo/interfaces/media_renderer.mojom b/media/mojo/interfaces/media_renderer.mojom
index db7adc8dd2cb1670d19a39e839dddca045c2e6a4..6b7e86e149b0f99c23ad80ec0719656f98bf6f43 100644
--- a/media/mojo/interfaces/media_renderer.mojom
+++ b/media/mojo/interfaces/media_renderer.mojom
@@ -9,10 +9,11 @@ import "media/mojo/interfaces/media_types.mojom";
 
 [Client=MediaRendererClient]
 interface MediaRenderer {
-  // Initializes the Renderer with |stream|, calling back upon completion.
+  // Initializes the Renderer with one or both of an audio and video stream,
+  // calling back upon completion.
   // NOTE: If an error occurs, MediaRendererClient::OnError() will be called
   // before the callback is executed.
-  Initialize(DemuxerStream stream) => ();
+  Initialize(DemuxerStream? audio, DemuxerStream? video) => ();
 
   // Discards any buffered data, executing callback when completed.
   // NOTE: If an error occurs, MediaRendererClient::OnError() can be called