Fix tab_switching measurement bug.

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.
 """
 
+import json
 import time
 
 from telemetry.core import util
 from telemetry.page import page_test
 from telemetry.value import histogram
 from telemetry.value import histogram_util
 
 from metrics import keychain_metric
 from metrics import power
 
@@ skipping 32 matching lines @@
 
     return browser.tabs.New()
 
   def StopBrowserAfterPage(self, browser, page):
     # Restart the browser after the last page in the pageset.
     return len(browser.tabs) >= len(page.page_set.pages)
 
   def ValidateAndMeasurePage(self, page, tab, results):
     """On the last tab, cycle through each tab that was opened and then record
     a single histogram for the tab switching metric."""
-    if len(tab.browser.tabs) != len(page.page_set.pages):
+    browser = tab.browser
+    if len(browser.tabs) != len(page.page_set.pages):
       return
 
+    if browser.tabs < 2:
+      raise Exception('Should have at least two tabs for tab switching')
+
     # Measure power usage of tabs after quiescence.
     util.WaitFor(tab.HasReachedQuiescence, 60)
 
-    if tab.browser.platform.CanMonitorPower():
+    if browser.platform.CanMonitorPower():
       self._power_metric.Start(page, tab)
       time.sleep(TabSwitching.SAMPLE_TIME)
       self._power_metric.Stop(page, tab)
       self._power_metric.AddResults(tab, results,)
 
     histogram_name = 'MPArch.RWH_TabSwitchPaintDuration'
     histogram_type = histogram_util.BROWSER_HISTOGRAM
     display_name = 'MPArch_RWH_TabSwitchPaintDuration'
     first_histogram = histogram_util.GetHistogram(
         histogram_type, histogram_name, tab)
     prev_histogram = first_histogram
 
-    for t in tab.browser.tabs:
-      t.Activate()
+    for tab_to_switch in browser.tabs:
+      tab_to_switch.Activate()
       def _IsDone():
         cur_histogram = histogram_util.GetHistogram(
-            histogram_type, histogram_name, tab)
+            histogram_type, histogram_name, tab_to_switch)
         diff_histogram = histogram_util.SubtractHistogram(
             cur_histogram, prev_histogram)
-        return diff_histogram
+        # TODO(deanliao): Add SubtractHistogramRawValue to process histogram
+        #     object instead of JSON string.
+        diff_histogram_count = json.loads(diff_histogram).get('count', 0)
+        return diff_histogram_count > 0
       util.WaitFor(_IsDone, 30)
+
+      # We need to get histogram again instead of getting cur_histogram as
+      # variables modified inside inner function cannot be retrieved. However,
+      # inner function can see external scope's variables.
       prev_histogram = histogram_util.GetHistogram(
-          histogram_type, histogram_name, tab)
+          histogram_type, histogram_name, tab_to_switch)
 
-    last_histogram = histogram_util.GetHistogram(
-        histogram_type, histogram_name, tab)
-    diff_histogram = histogram_util.SubtractHistogram(last_histogram,
-        first_histogram)
-
+    last_histogram = prev_histogram
+    total_diff_histogram = histogram_util.SubtractHistogram(last_histogram,
+                                                            first_histogram)
     results.AddSummaryValue(
         histogram.HistogramValue(None, display_name, 'ms',
-                                 raw_value_json=diff_histogram,
+                                 raw_value_json=total_diff_histogram,
                                  important=False))
 
     keychain_metric.KeychainMetric().AddResults(tab, results)