Fixes MipMapTest on Intel/OpenGL.

BUG=angleproject:1038

Change-Id: I19a0704969ef03d5d2ee3c9092daf4bd3a9818f7
Reviewed-on: https://chromium-review.googlesource.com/274818Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>