GLES specifies an error if BeginTransformFeedback is called when no
binding points would be used.

BUG=angleproject:2184
TEST=angle_end2end_tests

Change-Id: Ie4489b5ba63885e718dafdcdaacc02b603959be3
Reviewed-on: https://chromium-review.googlesource.com/719136Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>