Read the proper uniform location instead of assuming location 0. Fixes MipMapTest on Intel/OpenGL. BUG=angleproject:1038 Change-Id: I19a0704969ef03d5d2ee3c9092daf4bd3a9818f7 Reviewed-on: https://chromium-review.googlesource.com/274818 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>