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

tools/perf/measurements/webrtc.py

 # Copyright 2014 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.
 
 from metrics import cpu
 from metrics import memory
 from metrics import power
 from telemetry.page import page_measurement
 
 
 class WebRTC(page_measurement.PageMeasurement):
   """Gathers WebRTC-related metrics on a page set."""
 
   def __init__(self):
     super(WebRTC, self).__init__('RunWebrtc')
     self._cpu_metric = None
     self._memory_metric = None
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
+
+  def WillStartBrowser(self, browser):
+    self._power_metric = power.PowerMetric(browser)
 
   def DidStartBrowser(self, browser):
     self._cpu_metric = cpu.CpuMetric(browser)
     self._memory_metric = memory.MemoryMetric(browser)
 
   def DidNavigateToPage(self, page, tab):
     self._cpu_metric.Start(page, tab)
     self._memory_metric.Start(page, tab)
     self._power_metric.Start(page, tab)
 
   def CustomizeBrowserOptions(self, options):
     memory.MemoryMetric.CustomizeBrowserOptions(options)
     options.AppendExtraBrowserArgs('--use-fake-device-for-media-stream')
     options.AppendExtraBrowserArgs('--use-fake-ui-for-media-stream')
     power.PowerMetric.CustomizeBrowserOptions(options)
 
   def MeasurePage(self, page, tab, results):
     """Measure the page's performance."""
     self._memory_metric.Stop(page, tab)
     self._memory_metric.AddResults(tab, results)
 
     self._cpu_metric.Stop(page, tab)
     self._cpu_metric.AddResults(tab, results)
 
     self._power_metric.Stop(page, tab)
     self._power_metric.AddResults(tab, results)