Make credential_path a page attribute instead of page_set attribute.

tools/perf/page_sets/gmail_compose_discard.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.
 
 import re
 
 from telemetry.page import page as page_module
 from telemetry.page import page_set as page_set_module
 
 
 def _CreateXpathFunction(xpath):
   return ('document.evaluate("%s",'
                              'document,'
                              'null,'
                              'XPathResult.FIRST_ORDERED_NODE_TYPE,'
                              'null)'
           '.singleNodeValue' % re.escape(xpath))
 
 
 class GmailComposeDiscardPage(page_module.Page):
 
   """ Why: Compose and discard a new email """
 
   def __init__(self, page_set):
     super(GmailComposeDiscardPage, 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'
 
   def RunNavigateSteps(self, action_runner):
     action_runner.NavigateToPage(self)
     action_runner.WaitForJavaScriptCondition(
         'window.gmonkey !== undefined &&'
         'document.getElementById("gb") !== null')
 
   def ComposeClick(self, action_runner):
@@ skipping 20 matching lines @@
 
 
 class GmailComposeDiscardPageSet(page_set_module.PageSet):
 
   """
   Description: Gmail endure test: compose and discard an email.
   """
 
   def __init__(self):
     super(GmailComposeDiscardPageSet, self).__init__(
-      credentials_path='data/credentials.json',
       user_agent_type='desktop')
 
     self.AddPage(GmailComposeDiscardPage(self))