Adding Dawn stub backend to ANGLE Bug: angleproject:8473 Change-Id: I81ea83986e4e566169aa82250b5cfb727985ed01 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5201413 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Liza Burakova <liza@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>