Update perf page_set to use story_set

tools/perf/page_sets/tough_ad_cases.py

Index: tools/perf/page_sets/tough_ad_cases.py
diff --git a/tools/perf/page_sets/tough_ad_cases.py b/tools/perf/page_sets/tough_ad_cases.py
index 3706bfe5f66ef6df531203b4627988e460c15bef..dbc79a256b815d5f1f0c8321bb2e028630649309 100644
--- a/tools/perf/page_sets/tough_ad_cases.py
+++ b/tools/perf/page_sets/tough_ad_cases.py
@@ -2,8 +2,8 @@
 # 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 SwiffyPage(page_module.Page):
@@ -31,13 +31,13 @@ class SwiffyPage(page_module.Page):
       action_runner.Wait(10)
 
 
-class ToughAdCasesPageSet(page_set_module.PageSet):
+class ToughAdCasesPageSet(story.StorySet):
   """Pages for measuring performance with advertising content."""
 
   def __init__(self):
     super(ToughAdCasesPageSet, self).__init__(
         archive_data_file='data/tough_ad_cases.json',
-        bucket=page_set_module.INTERNAL_BUCKET)
+        cloud_storage_bucket=story.INTERNAL_BUCKET)
 
     base_url = 'http://localhost:8000'