[iOS] Fix app crash when backgrounding app.

Currently, the app crashes on foregrounding the app after presenting the
password suggestion sheet. This seems to be the result of improperly
implementing the applicationDidEnterBackground method. This CL fixes
this crash by adding the NSNotification argument to make the
@selector(applicationDidEnterBackground:) correctly call our background
handling method.

Bug: 1302267
Change-Id: Ib85a0a6661146b832671b711092b0b6ce5c29954
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3498184
Auto-Submit: Benjamin Williams <bwwilliams@google.com>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#977064}