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

tools/perf/benchmarks/sunspider.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.
 import collections
 import json
 import os
 
 from metrics import power
 from telemetry import benchmark
 from telemetry.page import page_measurement
 from telemetry.page import page_set
 
 
 _URL = 'http://www.webkit.org/perf/sunspider-1.0.2/sunspider-1.0.2/driver.html'
 
 
 class _SunspiderMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_SunspiderMeasurement, self).__init__()
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
 
   def CustomizeBrowserOptions(self, options):
     power.PowerMetric.CustomizeBrowserOptions(options)
 
+  def WillStartBrowser(self, browser):
+    self._power_metric = power.PowerMetric(browser)
+
   def DidNavigateToPage(self, page, tab):
     self._power_metric.Start(page, tab)
 
   def MeasurePage(self, page, tab, results):
     tab.WaitForJavaScriptExpression(
         'window.location.pathname.indexOf("results.html") >= 0'
         '&& typeof(output) != "undefined"', 300)
 
     self._power_metric.Stop(page, tab)
     self._power_metric.AddResults(tab, results)
@@ skipping 20 matching lines @@
   """Apple's SunSpider JavaScript benchmark."""
   test = _SunspiderMeasurement
 
   def CreatePageSet(self, options):
     ps = page_set.PageSet(
       archive_data_file='../page_sets/data/sunspider.json',
       make_javascript_deterministic=False,
       file_path=os.path.abspath(__file__))
     ps.AddPageWithDefaultRunNavigate(_URL)
     return ps