Vulkan: Add AtomicShared type In preparation for upcoming changes related to decoupling warmup of pre-rast and fragment shader subsets, create a new thread-safe version of vk::Shared so multiple threads can consume and safely destroy a shared Vulkan handle. Bug: angleproject:8601 Change-Id: I079e4186325537842456b7f2c7017f7e95ea7d54 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5361151 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: mohan maiya <m.maiya@samsung.com>