Make ProcessThread be a TaskQueue implementation

That would allow to switch components from relying on ProcessThreads to
relying on TaskQueue one by one, without introducing new threads.

Bug: webrtc:6289
Change-Id: I18fe5d679d4d4d0ddf4a11900c9814eb570284d6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/167533
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30631}