Vulkan: add wrapper for vkEvent Bug: angleproject:2908 Change-Id: Id6c9326f85f9ff17b963078f82bcf486bae66a83 Reviewed-on: https://chromium-review.googlesource.com/c/1296951 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>