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

tools/perf/benchmarks/dromaeo.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 os
 
 from metrics import power
 from telemetry import test
 from telemetry.core import util
 from telemetry.page import page_measurement
 from telemetry.page import page_set
 
 
 class _DromaeoMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_DromaeoMeasurement, 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.document.cookie.indexOf("__done=1") >= 0', 600)
 
     self._power_metric.Stop(page, tab)
     self._power_metric.AddResults(tab, results)
 
@@ skipping 109 matching lines @@
   """Dromaeo JSLib traverse jquery JavaScript benchmark"""
   tag = 'jslibtraversejquery'
   query_param = 'jslib-traverse-jquery'
 
 
 class DromaeoJslibTraversePrototype(_DromaeoBenchmark):
   """Dromaeo JSLib traverse prototype JavaScript benchmark"""
   tag = 'jslibtraverseprototype'
   query_param = 'jslib-traverse-prototype'