Add benchmark that imitates news reading.

tools/perf/page_sets/system_health/system_health_stories.py

 # Copyright 2016 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.system_health import single_page_stories
+from page_sets.system_health import platforms
+from page_sets.system_health import loading_stories
 
 from telemetry import story
 
 
 class _SystemHealthStorySet(story.StorySet):
   """User stories for the System Health Plan.
 
   See https://goo.gl/Jek2NL.
   """
   PLATFORM = NotImplemented
 
   def __init__(self, take_memory_measurement=True):
     super(_SystemHealthStorySet, self).__init__(
         archive_data_file=('../data/memory_system_health_%s.json' %
                            self.PLATFORM),
         cloud_storage_bucket=story.PARTNER_BUCKET)
-    for story_class in single_page_stories.IterAllStoryClasses():
+    for story_class in loading_stories.IterAllStoryClasses():
       if self.PLATFORM not in story_class.SUPPORTED_PLATFORMS:
         continue
       self.AddStory(story_class(self, take_memory_measurement))
 
 
 class DesktopSystemHealthStorySet(_SystemHealthStorySet):
   """Desktop user stories for Chrome System Health Plan."""
-  PLATFORM = 'desktop'
+  PLATFORM = platforms.DESKTOP
 
 
 class MobileSystemHealthStorySet(_SystemHealthStorySet):
   """Mobile user stories for Chrome System Health Plan."""
-  PLATFORM = 'mobile'
+  PLATFORM = platforms.MOBILE