Update perf page_set to use story_set

tools/perf/page_sets/tough_texture_upload_cases.py

Index: tools/perf/page_sets/tough_texture_upload_cases.py
diff --git a/tools/perf/page_sets/tough_texture_upload_cases.py b/tools/perf/page_sets/tough_texture_upload_cases.py
index 2a712ca9e50a5edeb81d9b1d5abd1c489acccbd9..cebf37d004990538147446967f33ba5ce3042ebe 100644
--- a/tools/perf/page_sets/tough_texture_upload_cases.py
+++ b/tools/perf/page_sets/tough_texture_upload_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 ToughTextureUploadCasesPage(page_module.Page):
@@ -19,7 +19,7 @@ class ToughTextureUploadCasesPage(page_module.Page):
       action_runner.Wait(10)
 
 
-class ToughTextureUploadCasesPageSet(page_set_module.PageSet):
+class ToughTextureUploadCasesPageSet(story.StorySet):
 
   """
   Description: A collection of texture upload performance tests
@@ -43,4 +43,3 @@ class ToughTextureUploadCasesPageSet(page_set_module.PageSet):
     ]
     for url in urls_list:
       self.AddUserStory(ToughTextureUploadCasesPage(url, self))
-