BUG=angle:701
TEST=webgl conformance tests

Change-Id: I13b35900956e7d840d5891a5bab90a400347cd6f
Reviewed-on: https://chromium-review.googlesource.com/209229Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>