Fix a CHECK in PasswordGenerationAgent.

It turns out that the current flow isn't working today
- Generate a new password.
- Have some JS to clear the field value.
- Type the username.

Chrome tries to presave an empty password. Instead it should detect
that the user isn't in the "generated" state anymore.


Fixed: 1454397
Change-Id: I11437dc32503091c777a0356db57b1e842ac7949
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4628290
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Ivana Žužić <izuzic@google.com>
Cr-Commit-Position: refs/heads/main@{#1160191}