Hardening NotificationView

There are some crashes that keep cropping up related
to dangling raw_ptr in notification view.

Invalidate weak ptrs during shutdown. This will prevent
callbacks using the weak_ptr_factory_ from executing during
destruction. I read through and found that this should prevent
atleast three callback from being called during shutdown.

I'm not confident this will fix all of the dangling raw pointers,
I'll submit another fix later if this doesn't nip them.

Bug: b/335902543
Change-Id: I9111f0ad43fa7b025f1415cfc3f4a3df6b7e6a75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6446803
Auto-Submit: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Andre Le <leandre@chromium.org>
Commit-Queue: Andre Le <leandre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1445601}