This Cl:
1. Adding a CastRemotingSender for media remoting.
2. Modeled after media::cast::FrameSender, but is different in that:
a) it's just used as an opaque, reliable transport and does not manage
a media encoder; and b) is meant to run in the browser process instead
of the render process to avoid extra cross-process hopping for the
media remoting use case.
3. Intended to be created via the cast.streaming.rtp private extension
API, which is to be used by the Media Router component extension.

BUG=630390,643964

Review-Url: https://codereview.chromium.org/2307653002
Cr-Commit-Position: refs/heads/master@{#418409}