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

tools/perf/benchmarks/robohornet_pro.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 Microsoft's RoboHornet Pro benchmark."""
 
 import os
 
 from metrics import power
 from telemetry import benchmark
 from telemetry.page import page_measurement
 from telemetry.page import page_set
 
 
 class _RobohornetProMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_RobohornetProMeasurement, 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.ExecuteJavaScript('ToggleRoboHornet()')
     tab.WaitForJavaScriptExpression(
         'document.getElementById("results").innerHTML.indexOf("Total") != -1',
         600)
 
     self._power_metric.Stop(page, tab)
     self._power_metric.AddResults(tab, results)
 
     result = int(tab.EvaluateJavaScript('stopTime - startTime'))
     results.Add('Total', 'ms', result)
 
 
 class RobohornetPro(benchmark.Benchmark):
   test = _RobohornetProMeasurement
 
   def CreatePageSet(self, options):
     ps = page_set.PageSet(
       archive_data_file='../page_sets/data/robohornet_pro.json',
       # Measurement require use of real Date.now() for measurement.
       make_javascript_deterministic=False,
       file_path=os.path.abspath(__file__))
     ps.AddPageWithDefaultRunNavigate(
       'http://ie.microsoft.com/testdrive/performance/robohornetpro/')
     return ps