[Telemetry] Move the deprecate decorator of form_based_credentials_backend to LoginNeeded method

telemetry/telemetry/internal/backends/form_based_credentials_backend.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 import logging
 
 from telemetry import decorators
 from telemetry.core import exceptions
 
 
-# pylint: disable=line-too-long
-@decorators.Deprecated(2017, 5, 5, 'Use the login helper modules in '
-                       'https://code.google.com/p/chromium/codesearch#chromium/src/tools/perf/page_sets/login_helpers/'
-                       ' instead.')
-# pylint: enable=line-too-long
 class FormBasedCredentialsBackend(object):
   def __init__(self):
     self._logged_in = False
 
   def IsAlreadyLoggedIn(self, tab):
     return tab.EvaluateJavaScript(self.logged_in_javascript)
 
   @property
   def credentials_type(self):
     raise NotImplementedError()
@@ skipping 49 matching lines @@
     tab.ExecuteJavaScript(email_id)
     tab.ExecuteJavaScript(password)
     if self.login_button_javascript:
       tab.ExecuteJavaScript(self.login_button_javascript)
     else:
       tab.ExecuteJavaScript(
           'document.getElementById("%s").submit();' % self.login_form_id)
     # Wait for the form element to disappear as confirmation of the navigation.
     action_runner.WaitForNavigate()
 
-
+  # pylint: disable=line-too-long
+  @decorators.Deprecated(2017, 5, 5,
+                         'FormBasedCredentialsBackend is deprecated. Use the '
+                         'login helper modules in '
+                         'https://code.google.com/p/chromium/codesearch#chromium/src/tools/perf/page_sets/login_helpers/'
+                         ' instead.')
+  # pylint: enable=line-too-long
   def LoginNeeded(self, tab, action_runner, config):
     """Logs in to a test account.
 
     Raises:
       RuntimeError: if could not get credential information.
     """
     if self._logged_in:
       return True
 
     if 'username' not in config or 'password' not in config:
@@ skipping 21 matching lines @@
           action_runner, tab, config['username'], config['password'])
 
       self._logged_in = True
       return True
     except exceptions.TimeoutException:
       logging.warning('Timed out while loading: %s', url)
       return False
 
   def LoginNoLongerNeeded(self, tab): # pylint: disable=unused-argument
     assert self._logged_in