These were causing assertions in several WebGL2 conformance tests.

BUG=angleproject:882
BUG=angleproject:1084
BUG=483282

Change-Id: I7651c359a528c95469dcbfb5da2715ae4616b49b
Reviewed-on: https://chromium-review.googlesource.com/286592Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>