Vulkan: Make Context::mLastSubmittedSerial to QueueSerial Right now Context::mLastSubmittedSerial and mLastFlushedSerial are all Serial type instead of QueueSerial. Most places we do need QueueSerial. This CL changes them to QueueSerial type. This simplifies and improves code readability. No actual functional change is expected with this CL. Bug: b/267806287 Change-Id: Ib00cae2637ee9ed95e5fc00060c63017b04e26c5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4219944 Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com>