[telemetry] Use startup_url to force a fresh browser for each page in TodoMVC

tools/perf/page_sets/todomvc.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 telemetry.page import page as page_module
 from telemetry.page import shared_page_state
 from telemetry import story
 
 
 URL_LIST = [
     ('Polymer', 'http://todomvc.com/examples/polymer'),
     ('AngularJS', 'http://todomvc.com/examples/angularjs'),
     ('React', 'http://todomvc.com/examples/react'),
     ('Backbone.js', 'http://todomvc.com/examples/backbone'),
     ('Ember.js', 'http://todomvc.com/examples/emberjs'),
     ('Closure', 'http://todomvc.com/examples/closure'),
     ('GWT', 'http://todomvc.com/examples/gwt'),
     ('Dart', 'http://todomvc.com/examples/vanilladart/build/web'),
     ('Vanilla JS', 'http://todomvc.com/examples/vanillajs'),
 ]
 
 INTERACTION_NAME = 'Interaction.AppLoad'
 
 
 class TodoMVCPage(page_module.Page):
 
   def __init__(self, url, page_set, name):
     super(TodoMVCPage, self).__init__(
         url=url, page_set=page_set, name=name,
+        startup_url='about:blank',
         shared_page_state_class=shared_page_state.SharedDesktopPageState)
     # TODO(jochen): This interaction does not include the
     # WindowProxy::initialize portion before the commit. To fix this, we'll
     # have to migrate to TBMv2.
     self.script_to_evaluate_on_commit = (
         'console.time("%s");' % INTERACTION_NAME)
 
   def RunPageInteractions(self, action_runner):
     action_runner.ExecuteJavaScript(
         """
@@ skipping 15 matching lines @@
 
   """ TodoMVC examples """
 
   def __init__(self):
     super(TodoMVCPageSet, self).__init__(
       archive_data_file='data/todomvc.json',
       cloud_storage_bucket=story.PUBLIC_BUCKET)
 
     for name, url in URL_LIST:
       self.AddStory(TodoMVCPage(url, self, name))