Descriptor pools need to live as long as the descriptor sets that are
allocated from them. Using Serials while building a command to judge a
pool's lifetime is prone to errors, since a command's Serial value
isn't known until the command is submitted, leading to deleting pools
too early relative to when the descriptor set is actually used.

This CL updates DescriptorPoolHelper to inherit from Resource, so the
descriptor pools can be retain()'ed. This allows the Resource's counter
to indicate that a pool is in use until the command's Serial is known
and can be recorded to indicate when the command completes. This
prevents descriptor pools from being destroyed before the command
completes (while the descriptor sets are still in use), or even before
the command has been submitted. Destroying a descriptor pool resets all
of the descriptors that were allocated from it, which can trigger a
variety of VVL errors depending on when it's erroneously performed.

This CL also adds the necessary retain() calls for the descriptor pools.
In particular, the pools need to be retained each time a cached
descriptor set that was allocated from it is re-used. This is relatively
simple with the current design, since we always clear the descriptor set
caches whenever a new pool is allocated, so the descriptor pool binding
is always accurate.

Bug: angleproject:5030
Test: MultithreadingTest::MultiContextDrawSmallDescriptorPools()
Change-Id: I5fdeeb46159448dfd679d7169e423048348be5ab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2437609
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>