Make credential_path a page attribute instead of page_set attribute.

tools/perf/page_sets/tough_layout_cases.py

 # Copyright 2014 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.
 from telemetry.page import page as page_module
 from telemetry.page import page_set as page_set_module
 
 
 class ToughLayoutCasesPage(page_module.Page):
 
   def __init__(self, url, page_set):
-    super(ToughLayoutCasesPage, self).__init__(url=url, page_set=page_set)
-    self.credentials_path = 'data/credentials.json'
+    super(ToughLayoutCasesPage, self).__init__(
+        url=url, page_set=page_set, credentials_path = 'data/credentials.json')
     self.user_agent_type = 'desktop'
     self.archive_data_file = 'data/tough_layout_cases.json'
 
   def RunSmoothness(self, action_runner):
     interaction = action_runner.BeginGestureInteraction(
         'ScrollAction', is_smooth=True)
     action_runner.ScrollPage()
     interaction.End()
 
 
 class ToughLayoutCasesPageSet(page_set_module.PageSet):
 
   """
   The slowest layouts observed in the alexa top 1 million sites in  July 2013.
   """
 
   def __init__(self):
     super(ToughLayoutCasesPageSet, self).__init__(
-      credentials_path='data/credentials.json',
       user_agent_type='desktop',
       archive_data_file='data/tough_layout_cases.json',
       bucket=page_set_module.PARTNER_BUCKET)
 
     urls_list = [
       'http://oilevent.com',
       'http://www.muzoboss.ru',
       'http://natunkantha.com',
       'http://www.mossiella.com',
       'http://bookish.com',
       'http://mydiyclub.com',
       'http://amarchoti.blogspot.com',
       'http://picarisimo.es',
       'http://chinaapache.com',
       'http://indoritel.com'
     ]
 
     for url in urls_list:
       self.AddPage(ToughLayoutCasesPage(url, self))