Update perf page_set to use story_set

tools/perf/page_sets/mse_cases.py

Index: tools/perf/page_sets/mse_cases.py
diff --git a/tools/perf/page_sets/mse_cases.py b/tools/perf/page_sets/mse_cases.py
index 1550fe937b4a809aa59ade0e205fb8626a0a007a..680f99c2fbee7212257e8b60d793b94db222144c 100644
--- a/tools/perf/page_sets/mse_cases.py
+++ b/tools/perf/page_sets/mse_cases.py
@@ -1,8 +1,8 @@
 # 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):
@@ -15,12 +15,12 @@ class MseCasesPage(page_module.Page):
     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',