[Autofill] Set FormFieldData::is_visible() on iOS

On Blink platforms, FormFieldData::is_visible() heuristically determines
visibility of an element.

On iOS, no such heuristic is implemented.

This CL makes FormFieldData::is_visible() an alias for
FormFieldData::is_focusable() on iOS.

Bug: 429596531
Change-Id: Ic476f535cc3d7bd1569aea603b85cdabf4a08ab2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6700245
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1482607}