[Telemetry] Power metric: subtract quiescent power draw from result

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 time
 
 from metrics import histogram_util
 from metrics import power
 from telemetry.core import util
 from telemetry.page import page_measurement
 from telemetry.value import histogram
 
 # TODO: Revisit this test once multitab support is finalized.
 
 class TabSwitching(page_measurement.PageMeasurement):
+
+  # Amount of time to measure, in seconds.
+  SAMPLE_TIME = 30
+
   def __init__(self):
     super(TabSwitching, self).__init__()
     self._first_page_in_pageset = True
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
 
   def CustomizeBrowserOptions(self, options):
     options.AppendExtraBrowserArgs([
         '--enable-stats-collection-bindings'
     ])
     power.PowerMetric.CustomizeBrowserOptions(options)
 
-  def DidStartBrowser(self, browser):
+  def WillStartBrowser(self, browser):
     self._first_page_in_pageset = True
+    self._power_metric = power.PowerMetric(browser, TabSwitching.SAMPLE_TIME)
 
   def TabForPage(self, page, browser):
     if self._first_page_in_pageset:
       # The initial browser window contains a single tab, navigate that tab
       # rather than creating a new one.
       self._first_page_in_pageset = False
       return browser.tabs[0]
 
     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 MeasurePage(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):
       return
 
     # Measure power usage of tabs after quiescence.
     util.WaitFor(tab.HasReachedQuiescence, 60)
 
     self._power_metric.Start(page, tab)
-    time.sleep(60)
+    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
 
@@ skipping 12 matching lines @@
 
     last_histogram = histogram_util.GetHistogram(
         histogram_type, histogram_name, tab)
     diff_histogram = histogram_util.SubtractHistogram(last_histogram,
         first_histogram)
 
     results.AddSummaryValue(
         histogram.HistogramValue(None, display_name, '',
                                  raw_value_json=diff_histogram,
                                  important=False))