Vulkan: Switch RefCountedDescriptorPoolBinding to use SharedPtr This mostly a clean up. RefCountedDescriptorPoolBinding is replaced with DescriptorPoolPointer, which is defined as SharedPtr<DescriptorPoolHelper>. It has more intuitive semantics to use. Bug: angleproject:372268711 Change-Id: I0397111b5228e896c1d226e00930851319d955a0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5938947 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Charlie Lao <cclao@google.com>