Add initial support for MediaRenderer API.

Add MediaRenderer API to xwalk. Most of the APIs are implemented and I have tested the basic ones already. Need to fix few more bugs.

BUG=XWALK-1195
```
if (callback == null || !isValidType(type))
  return;

var index = g_media_renderer_manager_listeners[type].indexOf(callback);
if (~index)
  g_media_renderer_manager_listeners[type].slice(index, 1);
```