Extract the appropriate parts of Dawn's third_party BUILD.gn for it, and
put them in their own BUILD.gn.
Change-Id: Iedfc11321ca5366499dfc8a1759d8aa08eb78931
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/237442
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
