Fix tab_switching to support flag --pageset-repeat properly

tools/perf/measurements/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.
 
 """The tab switching measurement.
 
 This measurement opens pages in different tabs. After all the tabs have opened,
 it cycles through each tab in sequence, and records a histogram of the time
 between when a tab was first requested to be shown, and when it was painted.
 Power usage is also measured.
 """
 
 from telemetry.page import legacy_page_test
 from telemetry.value import histogram
 from telemetry.value import histogram_util
 
 from metrics import keychain_metric
 
 
 class TabSwitching(legacy_page_test.LegacyPageTest):
   def __init__(self):
     super(TabSwitching, self).__init__()
     self._first_histogram = None
 
   def CustomizeBrowserOptions(self, options):
     keychain_metric.KeychainMetric.CustomizeBrowserOptions(options)
 
     options.AppendExtraBrowserArgs(['--enable-stats-collection-bindings'])
 
+  @property
+  def is_multi_tab_test(self):
+    return True
+
   @classmethod
   def _GetTabSwitchHistogram(cls, tab_to_switch):
     histogram_name = 'MPArch.RWH_TabSwitchPaintDuration'
     histogram_type = histogram_util.BROWSER_HISTOGRAM
     return histogram_util.GetHistogram(
         histogram_type, histogram_name, tab_to_switch)
 
   def DidNavigateToPage(self, page, tab):
     """record the starting histogram"""
     self._first_histogram = self._GetTabSwitchHistogram(tab)
 
   def ValidateAndMeasurePage(self, page, tab, results):
     """record the ending histogram for the tab switching metric."""
     last_histogram = self._GetTabSwitchHistogram(tab)
     total_diff_histogram = histogram_util.SubtractHistogram(last_histogram,
                             self._first_histogram)
 
     display_name = 'MPArch_RWH_TabSwitchPaintDuration'
     results.AddSummaryValue(
         histogram.HistogramValue(None, display_name, 'ms',
             raw_value_json=total_diff_histogram,
             important=False))
 
     keychain_metric.KeychainMetric().AddResults(tab, results)