Adds a script to help with adding or removing Dawn milestones.
Bug: chromium:1432491
Change-Id: I0882503f4ceb7ba3454e13ec1669c053055ac975
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/133165
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Brian Sheedy <bsheedy@google.com>
Reviewed-by: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>