Add CHECKs to try to identify source of null callbacks.

Not actually sure this will catch them but should be sane state to
enforce anyway.

Bug: 1447532
Change-Id: If64f38b9e943304a7e948651f46e1f2c5dd19ce7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4554124


Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148003}