Make credential_path a page attribute instead of page_set attribute.

tools/perf/page_sets/gmail_refresh.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 GmailRefreshPage(page_module.Page):
 
   """ Why: Continually reload the gmail page. """
 
   def __init__(self, page_set):
     super(GmailRefreshPage, self).__init__(
       url='https://mail.google.com/mail/',
-      page_set=page_set)
-
-    self.credentials_path = 'data/credentials.json'
+      page_set=page_set, credentials_path = 'data/credentials.json')
     self.credentials = 'google'
     self.user_agent_type = 'desktop'
     self.archive_data_file = 'data/gmail_refresh.json'
 
   def RunEndure(self, action_runner):
     action_runner.ReloadPage()
 
 
 class GmailRefreshPageSet(page_set_module.PageSet):
 
   """
   Description: Chrome Endure control test to test gmail page reload
   """
 
   def __init__(self):
     super(GmailRefreshPageSet, self).__init__(
-      credentials_path='data/credentials.json',
       user_agent_type='desktop',
       archive_data_file='data/gmail_refresh.json',
       bucket=page_set_module.PUBLIC_BUCKET)
 
     self.AddPage(GmailRefreshPage(self))