Instead of duplicating GL header define values, explictly return
GLenum for variable queries in the shader inspection API. This
reduces the duplicate defines in the shader compiler header.

BUG=angle:466

Change-Id: If631b20ce68747297a946e1371def7709027a613
Reviewed-on: https://chromium-review.googlesource.com/204937Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>