ES31: Enable shader storage buffer support for OpenGL backend 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>