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

At the start of the story set, open the browsers associated with all
stories. This makes sure that, when taking memory dumps, we always get
a dump for all browsers all the time.

BUG=616043

Review-Url: https://codereview.chromium.org/2073713003
Cr-Commit-Position: refs/heads/master@{#400667}
diff --git a/tools/perf/page_sets/dual_browser_story.py b/tools/perf/page_sets/dual_browser_story.py
index e511044..036a929 100644
--- a/tools/perf/page_sets/dual_browser_story.py
+++ b/tools/perf/page_sets/dual_browser_story.py
@@ -101,6 +101,11 @@
     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