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 (!media_renderers_.size())
  return MediaRendererPtr();

std::map<std::string, MediaRendererPtr>::const_iterator it;
if ((it = media_renderers_.find(id)) != media_renderers_.end())
  return (*it).second;

return MediaRendererPtr();
```