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

tools/perf/benchmarks/kraken.py

 # Copyright 2012 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.
 
 """Runs Mozilla's Kraken JavaScript benchmark."""
 
 import os
 
 from metrics import power
 from telemetry import benchmark
 from telemetry.page import page_measurement
 from telemetry.page import page_set
 
 
 def _Mean(l):
   return float(sum(l)) / len(l) if len(l) > 0 else 0.0
 
 
 class _KrakenMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_KrakenMeasurement, 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(
         'document.title.indexOf("Results") != -1', 700)
     tab.WaitForDocumentReadyStateToBeComplete()
 
     self._power_metric.Stop(page, tab)
     self._power_metric.AddResults(tab, results)
@@ skipping 16 matching lines @@
   """Mozilla's Kraken JavaScript benchmark."""
   test = _KrakenMeasurement
 
   def CreatePageSet(self, options):
     ps = page_set.PageSet(
       archive_data_file='../page_sets/data/kraken.json',
       file_path=os.path.abspath(__file__))
     ps.AddPageWithDefaultRunNavigate(
       'http://krakenbenchmark.mozilla.org/kraken-1.1/driver.html')
     return ps