Migrate from telemetry PageSet -> StorySet.

tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/synthetic.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
+from telemetry import story
 
 
 class SyntheticPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(SyntheticPage, self).__init__(url=url, page_set=page_set)
 
 
-class SyntheticPageSet(page_set_module.PageSet):
+class SyntheticStorySet(story.StorySet):
 
   """ Chrome proxy synthetic test pages. """
 
   def __init__(self):
-    super(SyntheticPageSet, self).__init__()
+    super(SyntheticStorySet, self).__init__()
 
     urls_list = [
       'http://check.googlezip.net/test.html',
       'http://check.googlezip.net/static/'
     ]
 
     for url in urls_list:
-      self.AddUserStory(SyntheticPage(url, self))
+      self.AddStory(SyntheticPage(url, self))