This enables the simple operations clear test on Vulkan. The current
implementation is very synchronous - it will block and finish the
current command buffer if there is any possibility of a race.

BUG=angleproject:1319

Change-Id: If01fe9a19ed6f539639a38786193d3626164cada
Reviewed-on: https://chromium-review.googlesource.com/367754
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>