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

tools/perf/measurements/image_decoding.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.
 
 from metrics import power
 from telemetry.page import page_measurement
 from telemetry.timeline import model
 
 
 class ImageDecoding(page_measurement.PageMeasurement):
   def __init__(self):
     super(ImageDecoding, self).__init__()
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
 
   def CustomizeBrowserOptions(self, options):
     options.AppendExtraBrowserArgs('--enable-gpu-benchmarking')
     power.PowerMetric.CustomizeBrowserOptions(options)
 
+  def WillStartBrowser(self, browser):
+    self._power_metric = power.PowerMetric(browser)
+
   def WillNavigateToPage(self, page, tab):
     tab.ExecuteJavaScript("""
         if (window.chrome &&
             chrome.gpuBenchmarking &&
             chrome.gpuBenchmarking.clearImageCache) {
           chrome.gpuBenchmarking.clearImageCache();
         }
     """)
     self._power_metric.Start(page, tab)
     # FIXME: bare 'devtools' is for compatibility with older reference versions
@@ skipping 35 matching lines @@
     assert durations, 'Failed to find "Decode Image" trace events.'
 
     image_decoding_avg = sum(durations) / len(durations)
     results.Add('ImageDecoding_avg', 'ms', image_decoding_avg)
     results.Add('ImageLoading_avg', 'ms',
                 tab.EvaluateJavaScript('averageLoadingTimeMs()'))
 
   def CleanUpAfterPage(self, page, tab):
     if tab.browser.is_tracing_running:
       tab.browser.StopTracing()