Compute shaders can be now compiled and linked to create programs.
Some tests are added to verify successful and unsuccessful compute
shader linking.

The patch also replaces std::array<int, 3> with a custom struct
WorkGroupSize.

BUG=angleproject:1442

TEST=angle_end2end_tests
TEST=angle_unittests

Change-Id: I4ab0ac05755d0167a6d2a798f8d7f1516cf54d84
Reviewed-on: https://chromium-review.googlesource.com/366740Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>