Based on kbr's patch:
https://chromium-review.googlesource.com/c/angle/angle/+/890605

Implements new rules in this revised WebGL conformance test:
https://github.com/KhronosGroup/WebGL/pull/2583

BUG=chromium:806557

Change-Id: I84701dd7156f0bc4a45ba68e63cb962d2d54c2e5
Reviewed-on: https://chromium-review.googlesource.com/952567
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>