perf: Speculative fix for Swiffy WebGL ad pages

tools/perf/page_sets/tough_ad_cases.py

 # Copyright 2015 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 import repeatable_synthesize_scroll_gesture_shared_state
 
+from telemetry.core import util
 from telemetry.page import page as page_module
 from telemetry import story
 
 
 class SwiffyPage(page_module.Page):
 
   def __init__(self, url, page_set):
     super(SwiffyPage, self).__init__(url=url, page_set=page_set,
                                      make_javascript_deterministic=False)
 
   def RunNavigateSteps(self, action_runner):
     super(SwiffyPage, self).RunNavigateSteps(action_runner)
+    # Make sure the ad has finished loading.
+    util.WaitFor(action_runner.tab.HasReachedQuiescence, 60)
     # Swiffy overwrites toString() to return a constant string, so "undo" that
     # here so that we don't think it has stomped over console.time.
     action_runner.EvaluateJavaScript(
         'Function.prototype.toString = function() { return "[native code]"; }')
     # Make sure we have a reasonable viewport for mobile.
     viewport_js = (
         'var meta = document.createElement("meta");'
         'meta.name = "viewport";'
         'meta.content = "width=device-width";'
         'document.getElementsByTagName("head")[0].appendChild(meta);')
@@ skipping 175 matching lines @@
     self.AddStory(AdPage('http://androidpolice.com', self, scroll=scroll,
         wait_for_interactive_or_better=True))
 
 
 class ScrollingToughAdCasesPageSet(ToughAdCasesPageSet):
   """Pages for measuring scrolling performance with advertising content."""
 
   def __init__(self):
     super(ScrollingToughAdCasesPageSet, self).__init__(
         scroll=True)