Vulkan: Fix RefCountedDescriptorPoolHelper leak DynamicDescriptorPool::mDescriptorPools really owns the pool, make it std::unique_ptr instead of plain c++ pointer to automatic handle object destruction. Bug: chromium:1346946 Change-Id: Iec2fff920d624cd983d314086cc08832c6c61984 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3795008 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Charlie Lao <cclao@google.com>