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..b8df6d31006620540dc09e9c0f35f3d814085b28 100644
--- a/tools/perf/page_sets/system_health/system_health_stories.py
+++ b/tools/perf/page_sets/system_health/system_health_stories.py
@@ -2,6 +2,7 @@
 # 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 platforms
 from page_sets.system_health import single_page_stories
 
 from telemetry import story
@@ -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