This brings CommandQueue one step closer to the worker thread.

Bug: b/172704839
Change-Id: I35225c5f302e34e3feb38a35c22b6d8e6f4ad10c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2524543Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>