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>