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

tools/perf/measurements/smoothness.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.
 
 from metrics import power
 from measurements import smoothness_controller
 from telemetry.page import page_measurement
 
 
 class Smoothness(page_measurement.PageMeasurement):
   def __init__(self):
     super(Smoothness, self).__init__('RunSmoothness')
     self._power_metric = None
     self._smoothness_controller = None
 
   @classmethod
   def CustomizeBrowserOptions(cls, options):
     options.AppendExtraBrowserArgs('--enable-gpu-benchmarking')
     options.AppendExtraBrowserArgs('--touch-events=enabled')
     power.PowerMetric.CustomizeBrowserOptions(options)
 
+  def WillStartBrowser(self, browser):
+    self._power_metric = power.PowerMetric(browser)
+
   def WillRunActions(self, page, tab):
-    self._power_metric = power.PowerMetric()
     self._power_metric.Start(page, tab)
     self._smoothness_controller = smoothness_controller.SmoothnessController()
     self._smoothness_controller.Start(page, tab)
 
   def DidRunActions(self, page, tab):
     self._power_metric.Stop(page, tab)
     self._smoothness_controller.Stop(tab)
 
   def MeasurePage(self, page, tab, results):
     self._power_metric.AddResults(tab, results)
     self._smoothness_controller.AddResults(tab, results)
 
   def CleanUpAfterPage(self, _, tab):
     self._smoothness_controller.CleanUp(tab)