Make credential_path a page attribute instead of page_set attribute.

tools/perf/page_sets/gmail_alt_two_labels.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
 
 
 def _GetCurrentLocation(action_runner):
   return action_runner.EvaluateJavaScript('document.location.href')
 
 
 def _WaitForLocationChange(action_runner, old_href):
   action_runner.WaitForJavaScriptCondition(
       'document.location.href != "%s"' % old_href)
 
 
 class GmailAltTwoLabelsPage(page_module.Page):
 
   """ Why: Alternate between Inbox and Sent Mail """
 
   def __init__(self, page_set):
     super(GmailAltTwoLabelsPage, self).__init__(
       url='https://mail.google.com/mail/',
       page_set=page_set,
-      name='gmail_alt_two_labels')
-
-    self.credentials_path = 'data/credentials.json'
+      name='gmail_alt_two_labels',
+      credentials_path = 'data/credentials.json')
     self.credentials = 'google'
     self.user_agent_type = 'desktop'
     self.archive_data_file = 'data/gmail_alt_two_labels.json'
 
   def RunNavigateSteps(self, action_runner):
     action_runner.NavigateToPage(self)
     action_runner.WaitForJavaScriptCondition(
         'window.gmonkey !== undefined && '
         'document.getElementById("gb") !== null')
 
   def RunEndure(self, action_runner):
     old_href = _GetCurrentLocation(action_runner)
     action_runner.ClickElement(
         'a[href="https://mail.google.com/mail/u/0/?shva=1#sent"]')
     _WaitForLocationChange(action_runner, old_href)
     action_runner.Wait(1)
     old_href = _GetCurrentLocation(action_runner)
     action_runner.ClickElement(
         'a[href="https://mail.google.com/mail/u/0/?shva=1#inbox"]')
     _WaitForLocationChange(action_runner, old_href)
     action_runner.Wait(1)
 
 
 class GmailAltTwoLabelsPageSet(page_set_module.PageSet):
 
   """ Chrome Endure test for GMail. """
 
   def __init__(self):
     super(GmailAltTwoLabelsPageSet, self).__init__(
-      credentials_path='data/credentials.json',
       user_agent_type='desktop',
       archive_data_file='data/gmail_alt_two_labels.json',
       bucket=page_set_module.PUBLIC_BUCKET)
 
     self.AddPage(GmailAltTwoLabelsPage(self))