Stop browse:social:twitter scrolling wrong element

tools/perf/page_sets/system_health/browsing_stories.py

Index: tools/perf/page_sets/system_health/browsing_stories.py
diff --git a/tools/perf/page_sets/system_health/browsing_stories.py b/tools/perf/page_sets/system_health/browsing_stories.py
index 2f4efe8734cb4a31414f11f48d9141ed60a2239c..0f3976b6deb1066bcfc1d9528d32f8601536409e 100644
--- a/tools/perf/page_sets/system_health/browsing_stories.py
+++ b/tools/perf/page_sets/system_health/browsing_stories.py
@@ -20,6 +20,8 @@ class _BrowsingStory(system_health_story.SystemHealthStory):
 
   IS_SINGLE_PAGE_APP = False
   ITEM_SELECTOR = NotImplemented
+  # Defaults to using the body element if not set.
+  CONTAINER_SELECTOR = None
   ABSTRACT_STORY = True
 
   def _WaitForNavigation(self, action_runner):
@@ -31,7 +33,9 @@ class _BrowsingStory(system_health_story.SystemHealthStory):
         self.ITEM_SELECTOR, index)
     # Only scrolls if element is not currently in viewport.
     action_runner.WaitForElement(element_function=item_selector)
-    action_runner.ScrollPageToElement(element_function=item_selector)
+    action_runner.ScrollPageToElement(
+        element_function=item_selector,
+        container_selector=self.CONTAINER_SELECTOR)
     self._ClickLink(action_runner, item_selector)
 
   def _ClickLink(self, action_runner, element_function):
@@ -194,6 +198,7 @@ class TwitterMobileStory(_NewsBrowsingStory):
   NAME = 'browse:social:twitter'
   URL = 'https://www.twitter.com/nasa'
   ITEM_SELECTOR = '.Tweet-text'
+  CONTAINER_SELECTOR = '.NavigationSheet'
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY