[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 math
 import os
 
 from metrics import power
 from telemetry import benchmark
 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.getElementById("pause").value == "Run"', 60)
 
     # Start spying on POST request that will report benchmark results, and
     # intercept result data.
     tab.ExecuteJavaScript('(function() {' +
@@ skipping 158 matching lines @@
   """Dromaeo JSLib traverse prototype JavaScript benchmark"""
   tag = 'jslibtraverseprototype'
   query_param = 'jslib-traverse-prototype'
 
 
 class DromaeoCSSQueryJquery(_DromaeoBenchmark):
   """Dromaeo CSS Query jquery JavaScript benchmark"""
   tag = 'cssqueryjquery'
   query_param = 'cssquery-jquery'