This will prevent incorrect modifications to outputs from being checked
into the repository. Requires a few changes to some generators.

Bug: angleproject:3227
Change-Id: I5285cb78a9d85df155a5272edf8b6b8cd27fc04c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1515212Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>