Update perf page_set to use story_set

tools/perf/page_sets/key_hit_test_cases.py

Index: tools/perf/page_sets/key_hit_test_cases.py
diff --git a/tools/perf/page_sets/key_hit_test_cases.py b/tools/perf/page_sets/key_hit_test_cases.py
index e8a16690d92eaf9455ce108c0405b601eda85e08..b899e27f899cdcd46a71d839434e1c15604c20f8 100644
--- a/tools/perf/page_sets/key_hit_test_cases.py
+++ b/tools/perf/page_sets/key_hit_test_cases.py
@@ -2,9 +2,9 @@
 # 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
+
 
 class PaperCalculatorHitTest(polymer.PolymerPage):
 
@@ -37,7 +37,7 @@ class PaperCalculatorHitTest(polymer.PolymerPage):
       action_runner.TapElement(element_function='''window.__tapTarget''')
 
 
-class KeyHitTestCasesPageSet(page_set_module.PageSet):
+class KeyHitTestCasesPageSet(story.StorySet):
 
   def __init__(self):
     super(KeyHitTestCasesPageSet, self).__init__()