In replacing the legacy CommandQueue code with the threading
capable CommandProcessor it would be good to be able to run the
CommandProcessor in a single-thread environment. This CL changes the
meaning of the feature flags for the commandProcessor and
asynchronousCommandProcessing so that enabling commandProcess only
changes the code paths to use the command processor but work it still
done as part of the submitting thread (e.g. ContextVk).
Enabling asynchronousCommandProcessing will cause a separate worker
thread to be spawned which will asynchronously process the commands.
This allows us to switch to the CommandProcessor without threading and
then enable threading once performance issues are resolved.

Bug: b/161912801
Bug: b/170329600
Change-Id: I534862b109a7e7708108190b7c3e894071d4c2ed
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2483580Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>