Compute shaders were added to the core OpenGL spec in version 4.3.

Also don't output GL_ARB_explicit_attrib_location extension directive
for compute shaders. This extension is only relevant for vertex and
fragment shaders.

BUG=angleproject:1442
TEST=angle_end2end_tests

Change-Id: I92021ac048b38ffbc5a61af408b37c387193389b
Reviewed-on: https://chromium-review.googlesource.com/418657Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>