Move all test-related build stuff into its own BUILD.gn file. This
required moving the dawn_generator template into a common file, so it
can be called by both BUILD.gn and src/tests/BUILD.gn.
Bug: dawn:61
Change-Id: Icaa459270bdaa60306e053b93835812e70dba6f5
Reviewed-on: https://dawn-review.googlesource.com/c/2940
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Stephen White