Sequences of many frames with uniform updates could lead to a bug where
we attempt to delete descriptor sets that are still in use. To trigger
the bug we need to write enough uniform data to trigger a descriptor
set to be freed. This would correctly trigger refresh sets in the
Program. But if there was a second program idle in the background that
also allocated descriptors from the old pool, the bug would manifest.

Fix this by storing a shared handle to the descriptor pool in the
Program. The dynamic descriptor pool won't recycle descriptor pools
internally unless there are zero outstanding references to the pool.

We could also improve this in a resource sharing situation by keeping
a single shared dynamic descriptor pool per share group.

Includes a contribution from tobine@google.com that adds a test to
cover the bug.

Bug: angleproject:2863
Change-Id: Id585b85f33f8cfa3772ceff3af512d1e4fb0b75a
Reviewed-on: https://chromium-review.googlesource.com/c/1271919Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>