Add background stories to MobileSystemHealth

tools/perf/page_sets/system_health/loading_stories.py

Index: tools/perf/page_sets/system_health/loading_stories.py
diff --git a/tools/perf/page_sets/system_health/loading_stories.py b/tools/perf/page_sets/system_health/loading_stories.py
index eae99f9b2fbeaf3b4a2f66685b4a1c9402ac815c..2984dcefad15d9dd32fd68a3dfd91ec96c405669 100644
--- a/tools/perf/page_sets/system_health/loading_stories.py
+++ b/tools/perf/page_sets/system_health/loading_stories.py
@@ -291,6 +291,7 @@ class _LoadGmailBaseStory(_LoadingStory):
     # navigate to a sub-URL to set up the session and hit the resulting
     # redirection loop. Afterwards, we can safely navigate to
     # https://mail.google.com.
+    action_runner.tab.WaitForDocumentReadyStateToBeComplete()

[perezju, 2016/10/06 13:32:42]

why do we need this now?

[hjd, 2016/10/06 15:27:35]

Otherwise we never end up seeing the Navigate below.
I'm not entirely sure why, it feels like we try to navigate before the page is
done loading or something and then the navigate gets lost.
I think it is caused by something on the page changing (the original Gmail story
no longer works with --use-live-sites).

[perezju, 2016/10/06 15:32:54]

Ok, sounds good. Maybe just double-check after recording that both the original
and the new story work.

     action_runner.Navigate(
         'https://mail.google.com/mail/mu/mp/872/trigger_redirection_loop')
     action_runner.tab.WaitForDocumentReadyStateToBeComplete()