[SystemHealth] Wait for element to exist before trying to scroll to it in browsing stories.

tools/perf/page_sets/system_health/browsing_stories.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # 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 system_health_story
 
 from page_sets.login_helpers import pinterest_login
 
 from telemetry import decorators
@@ skipping 12 matching lines @@
   ABSTRACT_STORY = True
 
   def _WaitForNavigation(self, action_runner):
     if not self.IS_SINGLE_PAGE_APP:
       action_runner.WaitForNavigate()
 
   def _NavigateToItem(self, action_runner, index):
     item_selector = 'document.querySelectorAll("%s")[%d]' % (
         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)
     self._ClickLink(action_runner, item_selector)
 
   def _ClickLink(self, action_runner, element_function):
     action_runner.WaitForElement(element_function=element_function)
     action_runner.ClickElement(element_function=element_function)
     self._WaitForNavigation(action_runner)
 
   def _NavigateBack(self, action_runner):
     action_runner.ExecuteJavaScript('window.history.back()')
@@ skipping 316 matching lines @@
       inner_save_function = 'document.querySelector(".nameAndIcons")'
       action_runner.WaitForElement(element_function=inner_save_function)
       action_runner.ClickElement(element_function=inner_save_function)
       action_runner.Wait(1)  # Wait to make navigation realistic.
 
     # Close selection.
     x_element_function = ('document.querySelector('
                           '".Button.borderless.close.visible")')
     action_runner.ClickElement(element_function=x_element_function)
     action_runner.Wait(1)  # Wait to make navigation realistic.