TEST=conformance2/rendering/blitframebuffer-test

BUG=angleproject:1990

Change-Id: I0caeaac824f1689867134f34f74e5ef2c2f1b016
Reviewed-on: https://chromium-review.googlesource.com/475990Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>