Update perf page_set to use story_set

tools/perf/page_sets/key_idle_power_cases.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 page_sets import android_screen_restoration_shared_state
-
+from telemetry import story
+from telemetry.page import page as page_module
 from telemetry.page import shared_page_state
-from telemetry.page import page as page_module
-from telemetry.page import page_set as page_set_module
 
 
 class KeyIdlePowerPage(page_module.Page):
 
   def __init__(self, url, page_set, turn_screen_off,
                shared_page_state_class=shared_page_state.SharedMobilePageState):
     super(KeyIdlePowerPage, self).__init__(
         url=url,
         page_set=page_set,
         shared_page_state_class=(android_screen_restoration_shared_state
@@ skipping 12 matching lines @@
       # the screen is turned off. Several seconds should be enough time for the
       # browser to "settle down" into an idle state.
       action_runner.Wait(2)
 
   def RunPageInteractions(self, action_runner):
     # The page interaction is simply waiting in an idle state.
     with action_runner.CreateInteraction('IdleWaiting'):
       action_runner.Wait(20)
 
 
-class KeyIdlePowerCasesPageSet(page_set_module.PageSet):
+class KeyIdlePowerCasesPageSet(story.StorySet):
 
   """ Key idle power cases """
 
   def __init__(self):
     super(KeyIdlePowerCasesPageSet, self).__init__()
 
     foreground_urls_list = [
       # Why: Ensure minimal activity for static, empty pages in the foreground.
       'file://key_idle_power_cases/blank.html',
     ]
 
     for url in foreground_urls_list:
       self.AddUserStory(KeyIdlePowerPage(url, self, False))
 
     background_urls_list = [
       # Why: Ensure animated GIFs aren't processed when Chrome is backgrounded.
       'file://key_idle_power_cases/animated-gif.html',
       # Why: Ensure CSS animations aren't processed when Chrome is backgrounded.
       'file://key_idle_power_cases/css-animation.html',
       # Why: Ensure rAF is suppressed when Chrome is backgrounded.
       'file://key_idle_power_cases/request-animation-frame.html',
       # Why: Ensure setTimeout is throttled when Chrome is backgrounded.
       'file://key_idle_power_cases/set-timeout.html',
     ]
 
     for url in background_urls_list:
       self.AddUserStory(KeyIdlePowerPage(url, self, True))