This will help address a bug in glReadPixels performed after a resize.

BUG=angleproject:1050

Change-Id: Ie204dec229582ec0e990c6f7fa22cfd44048d46e
Reviewed-on: https://chromium-review.googlesource.com/286250Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>