Our secondary command buffer pools were leaking memory every submit.
This change pops the secondary command buffer pool allocator scope
after commands are flushed during a submit.

Fixes the memory leak observable on any Vulkan application.

Bug: angleproject:4029
Bug: angleproject:4403
Change-Id: I3d0f8d53456336ef0b03c6d37425fb76c88d1551
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057349Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>