Support for empty labels in phone number heuristics.

This CL adds support for phone number inputs of 2 or 3 consecutive
input fields, where only the first field has a label.
They are classified as
<country code> <city and number> and
<country code> <area code> <number>, respectively.

Guarded behind AutofillEnableParsingEmptyPhoneNumberLabels.

Bug: 1311937
Change-Id: I88b7033863481a3071134b4e35286c18f0c6c0cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3663061


Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1011351}