This has been found necessary on the D3D11 backend on AMD GPUs, and on
the D3D9 backend on all GPUs.

Tested an earlier version of this patch overnight on one of the
affected Windows AMD bots; it eliminated the flakiness observed on one
of the WebGL conformance tests.

BUG=491419

Change-Id: I668493eb5ec109b5e0650c154dc97b786e0745bd
Reviewed-on: https://chromium-review.googlesource.com/278265Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>