Vulkan: Lock around handle counter. TSAN showed we could have a data race when multiple threads were releasing objects because of the singleton handle counter. Bug: angleproject:6714 Change-Id: I23b5d343bec421a663198e7efc30c78dab2bde8f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3288328 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>