Add UMA stats for SAML on Chrome OS

This CL adds three UMA stats for SAML on Chrome OS:
* Was the principals API used (as opposed to scraping)?
* If scraping was used, how many passwords were found?
* If scraping was used, could one of the passwords be verified?

BUG=340831
TEST=Manual

Review URL: https://codereview.chromium.org/158873009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250690 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250690