Vulkan: Make CommandPool a wrapped type. This faciliatates an upcoming change to treat all handle types uniformly with respect to releasing. BUG=angleproject:1684 Change-Id: I632262a57b3a447cf4999c28ab359fe931549576 Reviewed-on: https://chromium-review.googlesource.com/442674 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>