Migrate from telemetry PageSet -> StorySet.

tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/fallback_viaheader.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 FallbackViaHeaderPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(FallbackViaHeaderPage, self).__init__(url=url, page_set=page_set)
 
 
-class FallbackViaHeaderPageSet(page_set_module.PageSet):
+class FallbackViaHeaderStorySet(story.StorySet):
   """ Chrome proxy test sites """
 
   def __init__(self):
-    super(FallbackViaHeaderPageSet, self).__init__()
+    super(FallbackViaHeaderStorySet, self).__init__()
 
     urls_list = [
         'http://chromeproxy-test.appspot.com/default?respStatus=200',
         'http://chromeproxy-test.appspot.com/default?respStatus=413',
     ]
 
     for url in urls_list:
-      self.AddUserStory(FallbackViaHeaderPage(url, self))
+      self.AddStory(FallbackViaHeaderPage(url, self))