This gives CommandQueue and CommandProcessor the exact same interface.
This also moves the worker thread to be owned by CommandProcessor.

Bug: b/172704839
Change-Id: Ife439bcf52d923e01a6a2166e0caaffce14fd086
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2537235
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>