Vulkan: Use vk::SharedPtr for SharedDescriptorSetCacheKey This CL switches SharedDescriptorSetCacheKey from using c++ std::shared_ptr to our internal version of vk::SharedPtr. Also get rid of an extra pointer indirection that SharedDescriptorSetCacheKey is a reference counted of actual cache key instead of std::unique_ptr of cache key. Bug: angleproject:372268711 Change-Id: Id9af5070d24f67711d6decc3a30a260b8d4062d9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6036302 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com>