Vulkan: Minor cleanup to DynamicDescriptorPool. In preparation for future changes. Bug: angleproject:6776 Change-Id: I0a2ab004883ad482f4ed19075c5f5f4e9c451ae0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3437413 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>