Breaks dEQP-GLES2.functional.texture.specification.basic_copyteximage2d.2d_alpha

BUG=angleproject:1188

This reverts commit 31e60cf2.

Change-Id: Icf1fcd984832a813536fe63e385b3cf9f667d66b
Reviewed-on: https://chromium-review.googlesource.com/309801Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>