Return variable type queries using GLenum values.

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/204937
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
32 files changed