Following up the PR in [1], add the necessary methods etc to support
MediaCapabilities-like queries for PeerConnection encoding.  

See the DD in [2].

This bug is a sibling of  https://crbug.com/709181  that landed the 
capabilities querying for Media Recorder.

[1] https://github.com/WICG/media-capabilities/pull/31
[2] https://docs.google.com/document/d/1a8zgCshVneckD8ZIsdCO1mTzVfINgQJTQDyFyxQzi7I/edit?ts=58dea745#