The patch adds support for barriers in vertex, fragment and compute
shaders.

BUG:angleproject:1442
TEST:angle_unittests

Change-Id: Ic85c3337911851a93a3f56bd935774181600eddd
Reviewed-on: https://chromium-review.googlesource.com/380641
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>