Skip flushes when there is no work submitted since the last
flush.

Clone of geofflang's:
https://chromium-review.googlesource.com/2911879

Bug: chromium:1181068
Change-Id: Idd8a1c61b814d746bf1e0225cfc4ca375a7bb224
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2915675
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>