Add GLPlayerMsg and glplayer_channel in canvas_traits

GLPlayerMsg enum values are going to be the commands to send to
the glplayer_thread.

glplayer_channel mod is a copy of webgl_channel.

components/canvas_traits/lib.rs

@@ -15,6 +15,7 @@ extern crate serde;
 
 pub mod canvas;
 pub mod media;
+mod media_channel;
 #[macro_use]
 pub mod webgl;
 mod webgl_channel;