[dawn][headers] Adds webgpu-headers DEP and diff tool

- Adds a DEP to webgpu-headers.
- Adds a script that can be used to generate diffs between
  Dawn's generated webgpu.h and the upstream one from
  webgpu-headers.
- Commits a diff file between the headers to keep track
  of changes.
- Adds a PRESUBMIT hook to verify that when changes are
  made to Dawn's header, or the upstream header from a
  roll, that the author needs to explicitly include an
  updated diff file.

Bug: 378865841
Change-Id: I0232ef874d6a7a611e372bf8b9f45c2498547684
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/241136
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>