Use FieldRendererId instead of FieldRef in AutofillAgent previews

Autofill currently stores the previewed elements as references to DOM
elements. This is suboptimal because it allows autofill to hold
references to elements that get disconnected from the DOM.

There's a feature that is meant to fix this, and there has been two
attempts at rolling it out that failed due to unexpected regressions in
submission detection.

Since we never saw any filling regression, I believe that this change is
safe.

Bug: 40056157
Change-Id: Id22103ceaefdceca39237d465bc3fdce468097d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6239772
Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1418083}