[Autofill] Do not reparse form if no reparse is needed

ParseFormAsync() should only reparse a form only if
- the form wasn't known before the event or
- the form has changed in a way that affects the predictions.

crrev.com/c/6917620 erroneously moved this check after the parse
(namely to update_cache() in ParseFormsAsyncCommon()). That is,
ParseFormAsync() always reparsed the form but discarded the result
if the form didn't change sufficiently compared to the cached form.

This CL fixes that: if the newly seen FormData and the cached
FormStructure are sufficiently similar, no reparse is done and
ParseFormAsync() terminates synchronously.

Bug: 427787155, 40232021
Change-Id: Ibcef31aedfc0588ca61c18ee6a5029aa5fe5da7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6938245
Commit-Queue: Christoph Schwering <schwering@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1514146}