[Autofill] Fix AutofillField's current value during import

FormStructure::RetrieveFromCache() for kFormImport resets each
field's value to the empty string if that field
- hasn't changed its value (current value == initial value) and
- is not a <select>.

The idea is that if the user didn't change an <input>'s value, its
value carries no meaning and can be ignored for form import.

The downside of this mechanism is that it creates ambiguity of
FormFieldData::value() / AutofillField::value(): its semantics
depends on whether the owning FormStructure was created in the
context of a form submission or not.

Now that each AutofillField knows its initial value, we can defer
the decision whether a value is relevant for import and avoid this
ambiguity.

This CL therefore introduces AutofillField::value_for_import(), which
applies the filters from FormStructure::RetrieveFromCache(kFormImport) toAutofillField::value(kCurrent).

As a by-product, value_for_import() also inlines the checks that
give precedence to FormFieldData::selected_option()->text.

This change preserves equivalence with one exception: when an
<input> is first seen in the submission and thus doesn't exist in
the cached form, then
- the *old* behaviour does not reset the value because
  RetrieveFromCache() doesn't find an initial value, whereas the
- the *new* behaviour does reset the value because the
  value(kCurrent) == value(kInitial).
6% of Autofill's and 0% of Password Manager's captured-site tests
contain such fields (and PWM doesn't use value_for_import() anyway).
This behaviour change shouldn't matter in the real world because when
a field is manually filled or autofilled, it is added to the cache
(except under extreme conditions).

All changes are behind the feature kAutofillFixCurrentValueInImport.

Bug: 40227496, 40232021
Change-Id: Id2f87f40f3cb2d0a33df31316251b541d867d12c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5874041
Reviewed-by: Dominic Battré <battre@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1358540}