[tools/perf] Keep both browsers open from the start of dual browser test

tools/perf/page_sets/dual_browser_story.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.
 
 import collections
 import logging
 import re
 import sys
 import urllib
 
@@ skipping 83 matching lines @@
     if finder_options.use_live_sites:
       wpr_mode = wpr_modes.WPR_OFF
     elif browser_options.wpr_mode == wpr_modes.WPR_RECORD:
       wpr_mode = wpr_modes.WPR_RECORD
     else:
       wpr_mode = wpr_modes.WPR_REPLAY
 
     self.platform.network_controller.Open(wpr_mode,
                                           browser_options.extra_wpr_args)
 
+    # Make sure all browsers are open before starting.
+    for story in story_set:
+      if story.browser_type not in self._browsers:
+        self._SetCurrentBrowser(story.browser_type)
+
   @property
   def current_tab(self):
     return self._current_tab
 
   @property
   def platform(self):
     return self._platform
 
   def _CheckTestEnabled(self, test, possible_browser):
     should_skip, msg = decorators.ShouldSkip(test, possible_browser)
@@ skipping 124 matching lines @@
           url=GOOGLE_SEARCH + urllib.urlencode({'q': query}),
           browser_type='android-webview',
           phase='on_webview'))
 
       # Stories that run on the browser selected by command line options.
       self.AddStory(SinglePage(
           name=re.sub('\W+', '_', url),
           url=url,
           browser_type='default',
           phase='on_chrome'))