cros: Change how multi-profile is enabled.

- Remove finch control so that enterprise enrollment is no longer required;
- Change the default behavior for primary user to be "not-allowed" so that
  only user with a proper enterprise policy could start a multi-profile
  session;
- Remove cached default behavior so that the changing default actually
  has effect;

BUG=336268
TEST=Manual. Only user with proper policy can start a multiprofile session.

Review URL: https://codereview.chromium.org/142403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246152 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246152