Extend the BigJs benchmark: add a YouTube page.

tools/perf/page_sets/big_js.py

 # Copyright 2014 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 telemetry.page import page as page_module
 from telemetry.page import page_set as page_set_module
 
 class BigJsPageSet(page_set_module.PageSet):
 
   """ Sites which load and run big JavaScript files."""
 
   def __init__(self):
     super(BigJsPageSet, self).__init__(
       archive_data_file='data/big_js.json',
       bucket=page_set_module.PARTNER_BUCKET,
       user_agent_type='desktop')
 
-    # Page sets with only one page don't work well, since we end up reusing a
+    # www.foo.com is a dummy page to make it easier to run only one interesting
+    # page at a time. You can't just run the interesting page on its own: Page
+    # sets with only one page don't work well, since we end up reusing a
     # renderer all the time and it keeps its memory caches alive (see
-    # crbug.com/403735). Add a dummy second page here.
+    # crbug.com/403735).
     urls_list = [
-        'http://beta.unity3d.com/jonas/DT2/',
-        'http://www.foo.com',
+       'http://beta.unity3d.com/jonas/DT2/',
+       'https://www.youtube.com/watch?v=IJNR2EpS0jw',
+       'http://www.foo.com',
     ]
     for url in urls_list:
       self.AddUserStory(page_module.Page(url, self))