[Autofill] Only fill/preview autofillable elements

Before crrev.com/c/5318007, ApplyFormAction() checked if the triggering
element is autofillable. The check was removed the check together with
the triggering-element concept.

This CL adds the check for all elements.

For example, when `<input type=text>` changes to `<input type=button>`
while the Autofill popup is open, this check prevents that we "fill"
the button.

Bug: 326733459
Change-Id: I258b3dc2b346bc2c02b3e6635fde91610af2530d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5323078
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1265470}