This returns the uniform index that we assigned for default uniforms.
All the dependent structure offsets can be determined from the base
register, so we won't have to store uniform information in the shader
variable.

BUG=angle:466

Change-Id: I0dd05251e8dba00c20d09fd865dfb150de56738e
Reviewed-on: https://chromium-review.googlesource.com/207254Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>