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 a4f7ac017a91e3f7c29b686fda6eda86ea9e8ea6..9ab234f3e67d89d845c12d789313695f97edf75c 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
@@ -62,13 +63,17 @@ class TabSwitching(page_test.PageTest):
   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.story_set.stories):
+    browser = tab.browser
+    if len(browser.tabs) != len(page.story_set.stories):
       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)
@@ -81,26 +86,31 @@ class TabSwitching(page_test.PageTest):
         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)
-      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)
+      # 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_to_switch)
 
+    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)