Refactor small form pruning

Results of autofill heuristics are discarded for small forms unless
specific field types were classified. E.g. email addresses may appear
stand-alone in a <form> tag.

This CL is an attempt to make the code easier to read. It should not
include any behavior changes.

Bug: 1474308
Change-Id: I0e9e5da4154bf44aeef74e86649188401faeb044
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4830223
Commit-Queue: Dominic Battre <battre@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Norge Vizcay <vizcay@google.com>
Cr-Commit-Position: refs/heads/main@{#1191262}