The shader translator can return the assigned register for a
block via a new API. This will let us delete the member variable
in interface blocks for the register -- a nice thing for GLSL.

BUG=angle:466

Change-Id: I9bc38e0cd031e32f90787be42c2324fc7c79dbf9
Reviewed-on: https://chromium-review.googlesource.com/206828Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>