This changes puts descriptor pools in the BindGroupLayout so that they
can be reused between descriptors with the same layout.
This makes the DrawCallPerf.Run/Vulkan_NoReuseBindGroups benchmark go
from 1400ns to 800ns per run.
BUG=dawn::256
Change-Id: Ia9baf7f998d9ff4d552e255c80069b67c6a9ac40
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/12920
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
