[Password Autofill] Add UMA stats for suppressed passwords due to low score.

BUG=294468

Review URL: https://codereview.chromium.org/25015006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226124 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226124