BUG=angleproject:1951
TEST=angle_end2end_tests:ShaderStorageBuffer

Change-Id: I1afc3cd005ad2e595c6ce937fc53e17423f8ec8b
Reviewed-on: https://chromium-review.googlesource.com/618132
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>