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

tools/perf/benchmarks/octane.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 Octane 2.0 javascript benchmark.
 
 Octane 2.0 is a modern benchmark that measures a JavaScript engine's performance
 by running a suite of tests representative of today's complex and demanding web
 applications. Octane's goal is to measure the performance of JavaScript code
 found in large, real-world web applications.
 Octane 2.0 consists of 17 tests, four more than Octane v1.
 """
 
 import os
 
 from metrics import power
 from telemetry import benchmark
 from telemetry.page import page_measurement
 from telemetry.page import page_set
 from telemetry.util import statistics
 from telemetry.value import scalar
 
 _GB = 1024 * 1024 * 1024
 
 class _OctaneMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_OctaneMeasurement, 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 WillNavigateToPage(self, page, tab):
     if tab.browser.memory_stats['SystemTotalPhysicalMemory'] < 1 * _GB:
       skipBenchmarks = '"zlib"'
     else:
       skipBenchmarks = ''
     page.script_to_evaluate_on_commit = """
         var __results = [];
         var __real_log = window.console.log;
         window.console.log = function(msg) {
@@ skipping 37 matching lines @@
   test = _OctaneMeasurement
 
   def CreatePageSet(self, options):
     ps = page_set.PageSet(
       archive_data_file='../page_sets/data/octane.json',
       make_javascript_deterministic=False,
       file_path=os.path.abspath(__file__))
     ps.AddPageWithDefaultRunNavigate(
       'http://octane-benchmark.googlecode.com/svn/latest/index.html?auto=1')
     return ps