Vulkan: Make DynamicDescriptorPool single pool. This completes a prior refactor. DynamicDescriptorPool couldn't robustly handle multiple pools so only accept one as a parameter. We were already only using one at a time in practice. This removes the need to look up the pool index from the descriptor type which became problematic later on. Bug: angleproject:2717 Change-Id: I7fb92d5ab55bbe1f35b6b9cfcd8981cc1ca358f1 Reviewed-on: https://chromium-review.googlesource.com/1127157 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Luc Ferron <lucferron@chromium.org>