This adds an option to dawn_generator to generate files in a different
directory so the generated stay at the same place. Otherwise compilation
errors occur because of stale versions of the headers on the CQ
builders.
BUG=dawn:61
Change-Id: I71ceb3172b5a4e35911973a03be29d90fa684416
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/5304
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>