Do IWYU for base/strings/string_number_conversions.h in Autofill code

Files that use base::NumberToString() and friends should include
string_number_conversions.h directly instead of depending on transitive
includes from far away.

Change-Id: Ia0a59cb86c4b2c6a769d353b89c48425b0dd69d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6583265
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1466044}