Add benchmark that imitates news reading.

tools/perf/page_sets/system_health/system_health_stories.py

Index: tools/perf/page_sets/system_health/system_health_stories.py
diff --git a/tools/perf/page_sets/system_health/system_health_stories.py b/tools/perf/page_sets/system_health/system_health_stories.py
index 1defe24feb19d85d13811aff07076b2d1e1e228d..569d5c01d4973eaa83f5cd059c0bcc1e694d9f49 100644
--- a/tools/perf/page_sets/system_health/system_health_stories.py
+++ b/tools/perf/page_sets/system_health/system_health_stories.py
@@ -2,7 +2,8 @@
 # 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
 
@@ -19,7 +20,7 @@ class _SystemHealthStorySet(story.StorySet):
         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))
@@ -27,9 +28,9 @@ class _SystemHealthStorySet(story.StorySet):
 
 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