[Chromoting] Use WebrtcVideoEncoderSelector in WebrtcVideoStream

This change uses WebrtcVideoEncoderSelector to select a video encoder instead of
blindly select one based on SDP.

Bug: chromium:769391
Change-Id: I93db02a11b777a5c4b2aa1d19ac3938f29f42bca
Reviewed-on: https://chromium-review.googlesource.com/738891
Commit-Queue: Zijie He <[email protected]>
Reviewed-by: Lambros Lambrou <[email protected]>
Cr-Commit-Position: refs/heads/master@{#511922}
6 files changed