This bug slipped in with the GetUniform validation refactor.

BUG=angle:571

Change-Id: I2b87e6fe98224ba99c5b21a71d66b197fd618741
Reviewed-on: https://chromium-review.googlesource.com/214872Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>