Ensure callback is cancelled before being cleared or overwritten.

* Move some Cancels earlier and add Cancels to some paths.
* Set the "PendingResponse" values in a single place, with a Cancel
  immediately beforehand.
* Add DCHECK that callback is already disposed in DestroyOnDeviceState.

Also fixes an unrelated get->reset typo that fixes some histogram
logging.

Change-Id: If6f15290b6a556f4eb29377bfd6a0096775e1436
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420240
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282575}