Update perf page_set to use story_set

tools/perf/page_sets/mse_cases.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 import story
 from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 
 
 class MseCasesPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(MseCasesPage, self).__init__(url=url, page_set=page_set)
 
   def RunNavigateSteps(self, action_runner):
     super(MseCasesPage, self).RunNavigateSteps(action_runner)
     action_runner.WaitForJavaScriptCondition('window.__testDone == true')
 
 
-class MseCasesPageSet(page_set_module.PageSet):
+class MseCasesPageSet(story.StorySet):
 
   """ Media source extensions perf benchmark """
 
   def __init__(self):
-    super(MseCasesPageSet, self).__init__(bucket=page_set_module.PUBLIC_BUCKET)
+    super(MseCasesPageSet, self).__init__(cloud_storage_bucket=story.PUBLIC_BUCKET)
 
     urls_list = [
       'file://mse_cases/startup_test.html?testType=AV',
       'file://mse_cases/startup_test.html?testType=AV&useAppendStream=true',
       # pylint: disable=C0301
       'file://mse_cases/startup_test.html?testType=AV&doNotWaitForBodyOnLoad=true',
       # pylint: disable=C0301
       'file://mse_cases/startup_test.html?testType=AV&useAppendStream=true&doNotWaitForBodyOnLoad=true',
       'file://mse_cases/startup_test.html?testType=V',
       'file://mse_cases/startup_test.html?testType=V&useAppendStream=true',
       # pylint: disable=C0301
       'file://mse_cases/startup_test.html?testType=V&doNotWaitForBodyOnLoad=true',
       # pylint: disable=C0301
       'file://mse_cases/startup_test.html?testType=V&useAppendStream=true&doNotWaitForBodyOnLoad=true',
       'file://mse_cases/startup_test.html?testType=A',
       'file://mse_cases/startup_test.html?testType=A&useAppendStream=true',
       # pylint: disable=C0301
       'file://mse_cases/startup_test.html?testType=A&doNotWaitForBodyOnLoad=true',
       # pylint: disable=C0301
       'file://mse_cases/startup_test.html?testType=A&useAppendStream=true&doNotWaitForBodyOnLoad=true',
     ]
 
     for url in urls_list:
       self.AddUserStory(MseCasesPage(url, self))