Update perf page_set to use story_set

tools/perf/page_sets/throttled_plugins.py

 # Copyright 2015 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
 from telemetry.page import shared_page_state
-from telemetry.page import page
-from telemetry.page import page_set
 
 
-class ThrottledPluginsPageSet(page_set.PageSet):
+class ThrottledPluginsPageSet(story.StorySet):
   def __init__(self):
     super(ThrottledPluginsPageSet, self).__init__(
         archive_data_file='data/throttled_plugins.json',
-        bucket=page_set.PUBLIC_BUCKET)
+        cloud_storage_bucket=story.PUBLIC_BUCKET)
     urls_with_throttled_plugins = [
         'http://cnn.com',
         'http://techcrunch.com',
         'http://wsj.com',
         'http://youtube.com'
     ]
     for url in urls_with_throttled_plugins:
       self.AddUserStory(
           page.Page(
               url, page_set=self,
               shared_page_state_class=shared_page_state.SharedDesktopPageState))