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>