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
Refer to chromium naming rule, please substitute all `mediarenderer` with `media_renderer` include file names, target_name ,dir_name, etc. Sorry not give this comment in last round review.