Ensure callback is called if dialog is closed via system back

When the WARN dialog is closed via the system back, the copy or paste
action callback is not triggered. This can cause some issues, as certain
systems may find themselves in a problematic "intermediate" state.
Triggering the callbacks with null pointers is sufficient to "reset" the
system, essentially the same thing as the user acknowledging the warning
and cancelling the attempted clipboard action.

This also *technically* affects blocked actions too, but since blocked
actions are always blocked, in practice this has a much less visible
effect.

Copy actions are affected in that future copies don't work, as the copy
logic is rigid and will be stuck with a permanent `pending_writes > 0`.
So if a dialog is dismissed, future dialogs will still appear, but
attempting to "copy anyways" will just not work. This does also affect
blocked actions, but in practice all copies on that site will be
blocked, so this should never be observed.

Paste actions are more severely affected, with the entire tab freezing
up. Leaving and returning is possible and does fix things. This freezing
doesn't affect blocked actions, since there's already code in place
triggering a callback call to clear state.

Bug (copy):
https://screencast.googleplex.com/cast/NDc5MzAzODQ4NzQyMDkyOHwwNDc3NzJjMi0xMA

Bug (paste):
https://screencast.googleplex.com/cast/NDc0ODYwNTY3NTAxMjA5NnwxODc1MDAxMi05Yg

Fix (paste):
https://screencast.googleplex.com/cast/NTEyNjA1NDMxMzE5NzU2OHw4NTBlMGVmMy0zMw

Fix (copy):
https://screencast.googleplex.com/cast/NTY5MjEzMzMzNDg0MzM5Mnw1YjMwYjI2Yi04MQ

Bug: 434871993
Change-Id: I2d9dee442fd122f3a311cdc662af9b327ee959a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7003982
Reviewed-by: Nancy Xiao <nancylanxiao@google.com>
Commit-Queue: Charles Hager <clhager@google.com>
Cr-Commit-Position: refs/heads/main@{#1523920}