This will allow the Vulkan back-end to call Vulkan commands that return
VkResult within syncState.

Bug: angleproject:2717
Change-Id: I05e1379e71acaac56bcc9493c8561e82da000934
Reviewed-on: https://chromium-review.googlesource.com/1120150
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>