Remove mUniformBlockBindings and move its bindings to mUniformBlocks.

BUG=angleproject:1442

Change-Id: I62b4471990a44e626d2357c41cb914abc27cb18f
Reviewed-on: https://chromium-review.googlesource.com/532834Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>