This reverts commit 7a38f3f5.

Reason for revert: caused numerous test failures on FYI waterfall per http://crbug.com/1213507 .

Original change's description:
> GL: Skip redundant flushes.
>
> 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>

Bug: chromium:1181068
Bug: chromium:1213507
No-Try: True
Change-Id: Icd1662e4ea541375b44bddaa839a5241cdad5302
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2919503
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>