Vulkan: Add feature flag to use VkEvent for buffer barriers Bug: angleproject:360274928 Change-Id: Ie6e48a0a57c016571a1e94c0105a231c749c76f9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6180394 Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com>