Fix tab_switching measurement bug.

tools/perf/measurements/tab_switching.py

Index: tools/perf/measurements/tab_switching.py
diff --git a/tools/perf/measurements/tab_switching.py b/tools/perf/measurements/tab_switching.py
index d4110e17251a6b834de81b0acd198d631a5224bb..32243b4cdef424fa5f1dc48f609eabb19a3a1b4f 100644
--- a/tools/perf/measurements/tab_switching.py
+++ b/tools/perf/measurements/tab_switching.py
@@ -10,6 +10,7 @@ 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
@@ -59,49 +60,57 @@ class TabSwitching(page_test.PageTest):
     # 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):
+  def ValidateAndMeasurePage(self, page, last_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 = last_tab.browser
+    if len(browser.tabs) != len(page.page_set.pages):
       return
 
     # Measure power usage of tabs after quiescence.
-    util.WaitFor(tab.HasReachedQuiescence, 60)
+    util.WaitFor(last_tab.HasReachedQuiescence, 60)
 
-    if tab.browser.platform.CanMonitorPower():
-      self._power_metric.Start(page, tab)
+    if browser.platform.CanMonitorPower():
+      self._power_metric.Start(page, last_tab)
       time.sleep(TabSwitching.SAMPLE_TIME)
-      self._power_metric.Stop(page, tab)
-      self._power_metric.AddResults(tab, results,)
+      self._power_metric.Stop(page, last_tab)
+      self._power_metric.AddResults(last_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)
+        histogram_type, histogram_name, last_tab)
     prev_histogram = first_histogram
 
-    for i in xrange(len(tab.browser.tabs)):
-      t = tab.browser.tabs[i]
-      t.Activate()
+    for i in xrange(len(browser.tabs)):
+      # Note that browser.tabs iterable does not return tab object but tab id.
+      # (http://crbug.com/473202)
+      tab = browser.tabs[i]
+      tab.Activate()
       def _IsDone():
         cur_histogram = histogram_util.GetHistogram(
             histogram_type, histogram_name, tab)
         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)
 
-    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)
+    keychain_metric.KeychainMetric().AddResults(last_tab, results)