Update perf page_set to use story_set

tools/perf/page_sets/tough_memory_multi_tab.py

Index: tools/perf/page_sets/tough_memory_multi_tab.py
diff --git a/tools/perf/page_sets/tough_memory_multi_tab.py b/tools/perf/page_sets/tough_memory_multi_tab.py
index c2753063482e1c4e35c6e4b5a89840f17d29ece1..0e01dcd11eb724441d12bac3b8d12a1834850336 100644
--- a/tools/perf/page_sets/tough_memory_multi_tab.py
+++ b/tools/perf/page_sets/tough_memory_multi_tab.py
@@ -1,9 +1,9 @@
 # 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 shared_page_state
+from telemetry import story
 from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
+from telemetry.page import shared_page_state
 
 
 class ToughMemoryMultiTabPage(page_module.Page):
@@ -15,7 +15,7 @@ class ToughMemoryMultiTabPage(page_module.Page):
     self.archive_data_file = 'data/key_mobile_sites.json'
 
 
-class ToughMemoryMultiTabPageSet(page_set_module.PageSet):
+class ToughMemoryMultiTabPageSet(story.StorySet):
 
   """ Mobile sites for exercising multi-tab memory issues """