[Autofill] Eliminate AutofillDriverIOS::HandleParsedForms()

This CL replaces AutofillDriverIOS::HandleParsedForms() with an
AutofillManager::Observer of OnAfterFormsSeen().

This is equivalent because HandleParsedForms() each have only one
callsite, right after each other. Their only difference is that
HandleParsedForms() is only called only if `!forms.empty()`.
The new observer exactly replicates this behaviour.

The reason for eliminating HandleParsedForms() is that there is
no way to implement for non-iOS Chrome because FormStructures aren't
visible in the renderer.

Bug: 40276395
Change-Id: I453d175fba2824368beafbfb49ac079c872b7348
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5335367


Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1267405}