Like https://chromium-review.googlesource.com/317360, but for
code that's in ANGLE trunk but not yet rolled in by Chromium.

BUG=567877

Change-Id: I59b1568b134f57919eb333e99e7d185416d5d3b5
Reviewed-on: https://chromium-review.googlesource.com/317390Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Nico Weber <thakis@chromium.org>