Revert of [tools/perf] Keep both browsers open from the start of dual browser test (patchset #1 id:1 of https://codereview.chromium.org/2073713003/ )

Reason for revert:
Broke WPR on the benchmark

Original issue's description:
> [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
>
> Committed: https://crrev.com/f41edc4cf5d1b086ad00d70bf9fcb51d3fe9183e
> Cr-Commit-Position: refs/heads/master@{#400667}

TBR=nednguyen@google.com,primiano@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=616043

Review-Url: https://codereview.chromium.org/2084393002
Cr-Commit-Position: refs/heads/master@{#401264}
diff --git a/tools/perf/page_sets/dual_browser_story.py b/tools/perf/page_sets/dual_browser_story.py
index 94ae6d8..14412d635 100644
--- a/tools/perf/page_sets/dual_browser_story.py
+++ b/tools/perf/page_sets/dual_browser_story.py
@@ -101,11 +101,6 @@
     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