Update perf page_set to use story_set

tools/perf/page_sets/key_hit_test_cases.py

 # 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 page_set as page_set_module
+from page_sets import polymer
+from telemetry import story
 
-from page_sets import polymer
 
 class PaperCalculatorHitTest(polymer.PolymerPage):
 
   def __init__(self, page_set):
     super(PaperCalculatorHitTest, self).__init__(
       # Generated from https://github.com/zqureshi/paper-calculator
       # vulcanize --inline --strip paper-calculator/demo.html
       url='file://key_hit_test_cases/paper-calculator-no-rendering.html',
       page_set=page_set, run_no_page_interactions=False)
 
@@ skipping 12 matching lines @@
         'window.__tapTarget != null')
 
     for _ in xrange(100):
         self.TapButton(action_runner)
 
   def TapButton(self, action_runner):
     with action_runner.CreateInteraction('Action_TapAction'):
       action_runner.TapElement(element_function='''window.__tapTarget''')
 
 
-class KeyHitTestCasesPageSet(page_set_module.PageSet):
+class KeyHitTestCasesPageSet(story.StorySet):
 
   def __init__(self):
     super(KeyHitTestCasesPageSet, self).__init__()
 
     self.AddUserStory(PaperCalculatorHitTest(self))