Add a signin allowed policy which is used to determine if a user is allowed to...

Add a signin allowed policy which is used to determine if a user is allowed to sign in to Chrome. This will be used for example for one preference for managed user settings (Playpen project).

BUG=171370
TEST=unit_tests, browser_tests, manual

Review URL: https://chromiumcodereview.appspot.com/12088040

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@185359 0039d316-1c4b-4281-b951-d872f2087c98