Fix tab_switching to support flag --pageset-repeat properly

tools/perf/benchmarks/tab_switching.py

 # Copyright 2013 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 core import perf_benchmark
 
 from measurements import tab_switching
 import page_sets
 from telemetry import benchmark
 
@@ skipping 17 matching lines @@
     return page_sets.SystemHealthStorySet(platform='desktop',
                                           case='multitab:misc')
 
   @classmethod
   def Name(cls):
     return 'tab_switching.typical_25'
 
   @classmethod
   def ShouldTearDownStateAfterEachStoryRun(cls):
     return False
+
+  @classmethod
+  def ShouldTearDownStateAfterEachStorySetRun(cls):
+    return False