Fix that 0 is a valid memory barrier Bug: angleproject:2280 Change-Id: Iad82d5838a7efdb6f6287aafb9ab980e9e86468d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1496017 Reviewed-by: Jamie Madill (use @chromium please) <jmadill@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>