[web] Fix grouped autofill on iOS Chrome (#187459)

**Fixes #185327**

**Problem**

On iOS Chrome, Chrome Password Manager did put the saved password into
Flutter Web's hidden browser input. But Flutter Web still thought the
password field was empty, so when it refreshed the autofill form, it
copied that old empty value back into the browser input. The password
was erased before it showed up in the visible Flutter `TextField`.

**Fix**

Before copying Flutter's stored value into a non-focused autofill field,
Flutter Web now checks what is already in the browser input. If the
browser input already has a password that Flutter has not seen yet,
Flutter Web sends that password to the framework and leaves the browser
input alone instead of clearing it.

To avoid mistaking a deliberate app change for an autofill, it tracks
the last framework value for each field, carried
over when the autofill form is reused. A value the app changed
programmatically is still pushed into the DOM; only an
unchanged framework value sitting next to a changed DOM value is treated
as a browser autofill.

**Demo**

iOS Chrome:

* Before: <https://flutter-demo-34-before.web.app> - username fills,
password stays empty
* After: <https://flutter-demo-34-after.web.app> - username and password
both fill