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

tools/perf/page_sets/dual_browser_story.py

Index: tools/perf/page_sets/dual_browser_story.py
diff --git a/tools/perf/page_sets/dual_browser_story.py b/tools/perf/page_sets/dual_browser_story.py
index e511044f5dcf4e1a0f5d10fa6c7851feb313fd53..036a9297fd10d633a4b4bc7f24c863a41f6b0355 100644
--- a/tools/perf/page_sets/dual_browser_story.py
+++ b/tools/perf/page_sets/dual_browser_story.py
@@ -101,6 +101,11 @@ class MultiBrowserSharedState(story_module.SharedState):
     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