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

tools/perf/measurements/page_cycler.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.
 
 """The page cycler measurement.
 
 This measurement registers a window load handler in which is forces a layout and
 then records the value of performance.now(). This call to now() measures the
 time from navigationStart (immediately after the previous page's beforeunload
 event) until after the layout in the page's load event. In addition, two garbage
@@ skipping 21 matching lines @@
 
   def __init__(self, *args, **kwargs):
     super(PageCycler, self).__init__(*args, **kwargs)
 
     with open(os.path.join(os.path.dirname(__file__),
                            'page_cycler.js'), 'r') as f:
       self._page_cycler_js = f.read()
 
     self._speedindex_metric = speedindex.SpeedIndexMetric()
     self._memory_metric = None
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
     self._cpu_metric = None
     self._v8_object_stats_metric = None
     self._has_loaded_page = collections.defaultdict(int)
 
   @classmethod
   def AddCommandLineArgs(cls, parser):
     parser.add_option('--v8-object-stats',
         action='store_true',
         help='Enable detailed V8 object statistics.')
 
@@ skipping 21 matching lines @@
     if cold_runs_percent_set:
       number_warm_pageset_runs = int(
           (int(args.pageset_repeat) - 1) * (100 - args.cold_load_percent) / 100)
       number_warm_runs = number_warm_pageset_runs * args.page_repeat
       cls._cold_run_start_index = number_warm_runs + args.page_repeat
       cls.discard_first_result = (not args.cold_load_percent or
                                   cls.discard_first_result)
     else:
       cls._cold_run_start_index = args.pageset_repeat * args.page_repeat
 
+  def WillStartBrowser(self, browser):
+    """Initialize metrics once right before the browser has been launched."""

[qsr, 2014/07/01 09:02:24]

Nit: Why is there a comment on 2 of them, and not on the others?

+    self._power_metric = power.PowerMetric(browser)
+
   def DidStartBrowser(self, browser):
     """Initialize metrics once right after the browser has been launched."""
     self._memory_metric = memory.MemoryMetric(browser)
     self._cpu_metric = cpu.CpuMetric(browser)
     if self._record_v8_object_stats:
       self._v8_object_stats_metric = v8_object_stats.V8ObjectStatsMetric()
 
   def DidStartHTTPServer(self, tab):
     # Avoid paying for a cross-renderer navigation on the first page on legacy
     # page cyclers which use the filesystem.
@@ skipping 67 matching lines @@
     # preserve any initial profile cache for as long as possible.
     # The second is that, if we did cold runs first, we'd have a transition
     # page set during which we wanted the run for each URL to both
     # contribute to the cold data and warm the catch for the following
     # warm run, and clearing the cache before the load of the following
     # URL would eliminate the intended warmup for the previous URL.
     return (self._has_loaded_page[url] >= self._cold_run_start_index)
 
   def results_are_the_same_on_every_page(self):
     return False