This aligns ANGLE with the GL spec. With this fix, we pass
all of the dEQP conformance tests in Functional / Uniform
Buffer Object that don't rely on single buffers.

BUG=angle:507

Change-Id: I6063a416e3f7ee6d5327d96850e3d0cc8521c817
Reviewed-on: https://chromium-review.googlesource.com/213506Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>