[system health] Make mobile washington post story more robust.

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 b470d76985d31b54be685332b698a14ed8f15097..cb3e5efe8b894416cf7a2a4481f6735c665d9bf7 100644
--- a/tools/perf/page_sets/system_health/loading_stories.py
+++ b/tools/perf/page_sets/system_health/loading_stories.py
@@ -178,15 +178,18 @@ class LoadWashingtonPostMobileStory(_LoadingStory):
   URL = 'https://www.washingtonpost.com/pwa'
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY
   _CLOSE_BUTTON_SELECTOR = '.close'
+  _MAIL_BUTTON_SELECTOR = '.mailBlock'
 
   def _DidLoadDocument(self, action_runner):
     # Close the popup window. On Nexus 9 (and probably other tables) the popup
     # window does not have a "Close" button, instead it has only a "Send link
     # to phone" button. So on tablets we run with the popup window open. The
     # popup is transparent, so this is mostly an aesthetical issue.
-    has_button = action_runner.EvaluateJavaScript(
+    action_runner.WaitForElement(selector="%s, %s" %
+        (self._CLOSE_BUTTON_SELECTOR, self._MAIL_BUTTON_SELECTOR))
+    has_close_button = action_runner.EvaluateJavaScript(
         '!!document.querySelector("%s")' % self._CLOSE_BUTTON_SELECTOR)
-    if has_button:
+    if has_close_button:
       action_runner.ClickElement(selector=self._CLOSE_BUTTON_SELECTOR)