Add a password field count histogram for empty usernames investigation.

Add a password field count histogram for empty usernames investigation.

We already have the histogram PasswordManager.EmptyUsernames.TextAndPasswordFieldCount which reports the total count of username and password fields, but to better understand it, it would help to also have a separate count of only password fields.

BUG=456728
Committed: https://crrev.com/118baad5c78eb233c426336d996d4c546d6b057d
Cr-Commit-Position: refs/heads/master@{#333249}

[msramek, 2015-06-03 11:47:32]

msramek@chromium.org changed reviewers:
+ isherman@chromium.org, vasilii@chromium.org

[msramek, 2015-06-03 11:47:33]

Hi Ilya and Vasilii,

to the histogram that counts usernames and passwords in a password form, I'd
like to add a very similar one that counts only passwords.

Since we're doing this for password forms with no username, the difference in
counts will only appear in forms that start with a password field and then have
some plain text fields after that.

I want to see if this kind of form occurs often. If the two histograms differ
significantly, then that is indeed the case and I'll proceed to investigate in
this direction. If they don't differ, I'll just delete the first one histogram.

PTAL.

Thanks,
Martin

[vasilii, 2015-06-03 11:55:49]

lgtm

[Ilya Sherman, 2015-06-03 22:41:46]

LGTM

[msramek, 2015-06-08 07:04:14]

The CQ bit was checked by msramek@chromium.org

[commit-bot: I haz the power, 2015-06-08 07:04:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1149293007/1

[commit-bot: I haz the power, 2015-06-08 08:08:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-08 08:09:08]

Patchset 1 (id:??) landed as
https://crrev.com/118baad5c78eb233c426336d996d4c546d6b057d
Cr-Commit-Position: refs/heads/master@{#333249}