Update perf page_set to use story_set

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 import story
+from telemetry.page import page as page_module
 from telemetry.page import shared_page_state
-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))
 
@@ skipping 32 matching lines @@
     action_runner.WaitForElement(
         element_function=_CreateXpathFunction('//div[text()="COMPOSE"]'))
     self.ComposeClick(action_runner)
     action_runner.Wait(1)
     action_runner.WaitForElement(
         'div[class~="oh"][data-tooltip="Discard draft"]')
     action_runner.ClickElement('div[class~="oh"][data-tooltip="Discard draft"]')
     action_runner.Wait(1)
 
 
-class GmailComposeDiscardPageSet(page_set_module.PageSet):
+class GmailComposeDiscardPageSet(story.StorySet):
 
   """
   Description: Gmail endure test: compose and discard an email.
   """
 
   def __init__(self):
     super(GmailComposeDiscardPageSet, self).__init__()
 
     self.AddUserStory(GmailComposeDiscardPage(self))