This reverts commit 5dcd29a6.

Reason for revert: Breaking the ANGLE -> Chromium roller:

https://chromium-review.googlesource.com/c/chromium/src/+/2496281

Original change's description:
> Vulkan: Make DescriptorPoolHelper a Resource
>
> 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>

TBR=courtneygo@google.com,timvp@google.com,jmadill@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: angleproject:5030
Change-Id: I0fd6d9a0e1b0989b22368ef98652281288699deb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2497222
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>