Add reporting UsernameDetectionMethod from new parser

The old FormData -> PasswordForm parser reported to the
PasswordManager.UsernameDetectionMethod histogram, which method was used
to detect the username in the form.

This CL teaches the new parser to do the same.

The supporting enum is copied from the old one and modernised. There is
not point in attempting to share that code given that the old parser is
going away soon.

Bug: 845426
Change-Id: I7737cb8dc598c11b42c0e7e0f929afed5e830d7c
Reviewed-on: https://chromium-review.googlesource.com/1149866
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577841}
4 files changed