code: function(authResult, redirectUrl) {
          if (authResult.user) {
            handleSignedInUser(authResult.user);
          }
          if (authResult.additionalUserInfo) {
            document.getElementById('is-new-user').textContent =
                authResult.additionalUserInfo.isNewUser ?
                'New User' : 'Existing User';
          }
          // Do not redirect.
          return false;
        }
docs: Called when the user has been successfully signed in.