Vulkan: Free pool allocation leak with graph disabled. 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/+/2057349 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>