Remove dependency on ProcessThread.
Instead RtpStreamsSynchronizer uses the worker thread
and makes callbacks on the same thread. That in turn
simplifies locking for VideoReceiveStream2, which we'll
take advantage of later.
Bug: webrtc:11489
Change-Id: Id9a5a7977771b92e420a09cc472cfb43de5627cc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174221
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31200}
