BUG=angleproject:2290
TEST=dEQP-GLES31.functional.texture.multisample.samples_*.sample_position
TEST=TextureMultisampleTestES31.CheckSamplePositions*

Change-Id: If8b74c16d5c104215456e35b8922279be972cee3
Reviewed-on: https://chromium-review.googlesource.com/844062
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>