The generated mipmaps are still not correct on all ANGLE backends.

TEST=conformance/extensions/ext-sRGB

BUG=angleproject:2009

Change-Id: I6299dcb93c71f48e61ec660347b4747366ecfb71
Reviewed-on: https://chromium-review.googlesource.com/491367
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>