Notifications: Re-add weak_ptr checks when toggling expanded state.

For bug 957033, these weak_ptr checks were removed
(https://chromium.googlesource.com/chromium/src.git/+/5985d18020944cbab3e51d524c0c598d551dddda)
in favor of an async deletion fix. However, it looks like that fix wasn't complete and
old crashes started regressing, so we should restore these old weak_ptr checks.

BUG=965549, 950121, 957033

Change-Id: I4c443174af58c622ef317ae1fb9d603ad0cde62d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1622481
Commit-Queue: Tim Song <tengs@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662685}
3 files changed