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

telemetry/telemetry/internal/backends/form_based_credentials_backend.py

Index: telemetry/telemetry/internal/backends/form_based_credentials_backend.py
diff --git a/telemetry/telemetry/internal/backends/form_based_credentials_backend.py b/telemetry/telemetry/internal/backends/form_based_credentials_backend.py
index 15b638101aa36aa1bec142dc54f7cc6e5c5c3d60..c2c23dc69315de2e25be88dcc86fd990a8952264 100644
--- a/telemetry/telemetry/internal/backends/form_based_credentials_backend.py
+++ b/telemetry/telemetry/internal/backends/form_based_credentials_backend.py
@@ -7,11 +7,6 @@ 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
@@ -81,7 +76,13 @@ class FormBasedCredentialsBackend(object):
     # 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.