This clarified that we're using the Pool allocator/deallocator for
this type.

BUG=angleproject:993

Change-Id: If8c95f6054d07291e7014be0d4e35766ba2e943b
Reviewed-on: https://chromium-review.googlesource.com/269131Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>