Update perf page_set to use story_set

tools/perf/page_sets/webrtc_cases.py

Index: tools/perf/page_sets/webrtc_cases.py
diff --git a/tools/perf/page_sets/webrtc_cases.py b/tools/perf/page_sets/webrtc_cases.py
index 41010b84384567f6c44c4ebef3c63a1575a73ab5..63d1edb1372f86dec2548037a3d773ab917c514d 100644
--- a/tools/perf/page_sets/webrtc_cases.py
+++ b/tools/perf/page_sets/webrtc_cases.py
@@ -3,9 +3,8 @@
 # found in the LICENSE file.
 import os
 
+from telemetry import story
 from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
-
 
 WEBRTC_GITHUB_SAMPLES_URL = 'http://webrtc.github.io/samples/src/content/'
 
@@ -170,14 +169,14 @@ class Page9(WebrtcCasesPage):
     action_runner.Wait(30)
 
 
-class WebrtcCasesPageSet(page_set_module.PageSet):
+class WebrtcCasesPageSet(story.StorySet):
 
   """ WebRTC tests for Real-time audio and video communication. """
 
   def __init__(self):
     super(WebrtcCasesPageSet, self).__init__(
       archive_data_file='data/webrtc_cases.json',
-      bucket=page_set_module.PUBLIC_BUCKET)
+      cloud_storage_bucket=story.PUBLIC_BUCKET)
 
     self.AddUserStory(Page1(self))
     self.AddUserStory(Page2(self))