Some callers of queueSubmitOneOff require that the command being queued
to have been sent to the GPU. The new ensureSubmission parameter
indicates that behavior and when running with threaded worker will wait
for the worker queue to empty before returning.

Bug: b/170312581
Change-Id: Ib620fb37f4b9b4431451ccbd10807c0dff1842af
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2579041Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>