While working on https://chromium-review.googlesource.com/c/chromium/src/+/663104, I noticed that the CancelableTaskTracker does not support delayed tasks. So instead I had to track CancelableCallbacks manually. The alternative isn't too bad, but I thought it would be nice for CancelableTaskTracker to support delayed tasks for consistency with task runner API.

It looks like there is at least one more place that would like this [1]. There was an attempt to add support for delayed tasks, but it seems abandoned [2].

[1] https://cs.chromium.org/chromium/src/media/midi/midi_scheduler.cc?q=cancelabletasktracker+delayed&sq=package:chromium&dr=C&l=27
[2] https://chromiumcodereview.appspot.com/1048373003/