Upgrade remoting WebRTC host to Unified Plan

This sets SdpSemantics::kUnifiedPlan for the PeerConnection, and updates
the code to use the PeerConnection [Add/Remove]Track() APIs, as the old
Stream-based APIs cannot be used with Unified Plan.

Verified (with Chrome browser) that b/w caps and estimation work as
expected for direct and relayed connections. Also verified that audio is
still encoded and received in stereo at ~160kbps.

Bug: 903012
Change-Id: I1d165738664ef819b1ed7c52d016d95bc6f5e2fc
Reviewed-on: https://chromium-review.googlesource.com/c/1396839
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623997}
5 files changed