[remoting WebRTC host] Poll for connection route changes

This gets the RTCStats for the connection at regular intervals, and
determines the route type of the connection. If the connection is
upgraded from relayed (TURN) to direct/STUN, the bitrate caps are
updated, allowing the connection to use the full available b/w.

A recent server-side change made this scenario occur more frequently
(initial relayed connection that gets improved to STUN), so this CL
will greatly improve performance in this situation.

Bug: 887789
Change-Id: I172c03cb7a1f28b3d802db07bcae95a1e73a303e
Reviewed-on: https://chromium-review.googlesource.com/c/1428401
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625027}
2 files changed