Add background stories to MobileSystemHealth

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 10933b4690362ef0eee283e271ee32c033e102b9..fc41494fa5de439651a0dd3b87682f8c9204633b 100644
--- a/tools/perf/page_sets/system_health/system_health_stories.py
+++ b/tools/perf/page_sets/system_health/system_health_stories.py
@@ -16,7 +16,8 @@ class SystemHealthStorySet(story.StorySet):
 
   See https://goo.gl/Jek2NL.
   """
-  def __init__(self, platform, case=None, take_memory_measurement=False):
+  def __init__(self, platform, case=None, take_memory_measurement=False,
+               measure_in_background=False):
     super(SystemHealthStorySet, self).__init__(
         archive_data_file=('../data/system_health_%s.json' % platform),
         cloud_storage_bucket=story.PARTNER_BUCKET)
@@ -28,7 +29,8 @@ class SystemHealthStorySet(story.StorySet):
           platform not in story_class.SUPPORTED_PLATFORMS or
           case and not story_class.NAME.startswith(case + ':')):
         continue
-      self.AddStory(story_class(self, take_memory_measurement))
+      self.AddStory(
+          story_class(self, take_memory_measurement, measure_in_background))
 
 
 class DesktopSystemHealthStorySet(SystemHealthStorySet):