See e.g. https://chromium-review.googlesource.com/c/chromium/src/+/899446 -- this change only changes compiler flags if is_chromeos is true and has no effect at all otherwise.

If the patch touches .gn files, `gn analyze` could generate ninja files with and without patch into a temp dir (running gn is fast), and then early-out if the output is identical. This would safely allow us to do nothing for things like:

- compiler changes on just one platform
- new files added on just one platform


(If the patch touches non-BUILD.gn files too, the current analyze behavior needs to run too of course; this is just a hand-wavy idea. But it feels like it could work and like it could save some amount of comp resources>