ES31: Add atomic memory functions BUG=angleproject:1442 TEST=angle_unittests, angle_end2end_tests dEQP-GLES31.functional.compute.shared_var.atomic* dEQP-GLES31.functional.compute.basic.shared_atomic_op* dEQP-GLES31.functional.ssbo.atomic.* Change-Id: I82b54fde3a852d3bd917b1e19680baa1c28fce4d Reviewed-on: https://chromium-review.googlesource.com/765061 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org>