This will allow follow up commits to use them from the different parts
of the split-off BUILD.gn file.
BUG=dawn:61
Change-Id: I767f6c97c06ee73290d3df482880d1a2df92e187
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/5301
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
