Add histograms for whether the OS password is blank or not

BUG=303113

Review URL: https://codereview.chromium.org/114893004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240950 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240950