No need to do this in one shot (also tzik@'s making some bulk changes with his rewriter for simple patterns), but there're some easier ones that can also contribute to cleaner code:
E.g. ServiceWorkerVersion::RunAfterStartWorker can take OnceClosure instead of Closure.
Note that non-once closure/callback can be implicitly converted to that of once version (i.e. caller can keep calling the code with non-once versions at least during the transition), while the opposite is not true.
https://chromium.googlesource.com/chromium/src/+/lkcr/docs/callback.md
Comment 1 by falken@chromium.org
, Aug 17 2017