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

tools/perf/measurements/memory.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 memory
 from metrics import power
 from telemetry.page import page_measurement
 
 class Memory(page_measurement.PageMeasurement):
   def __init__(self):
     super(Memory, self).__init__('RunStressMemory')
     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._memory_metric = memory.MemoryMetric(browser)
 
   def DidNavigateToPage(self, page, tab):
     self._memory_metric.Start(page, tab)
     self._power_metric.Start(page, tab)
 
   def CustomizeBrowserOptions(self, options):
     memory.MemoryMetric.CustomizeBrowserOptions(options)
@@ skipping 11 matching lines @@
       # The tcmalloc_heap_profiler dumps files at regular
       # intervals (~20 secs).
       # This is a minor optimization to ensure it'll dump the last file when
       # the test completes.
       tab.ExecuteJavaScript("""
         if (chrome && chrome.memoryBenchmarking) {
           chrome.memoryBenchmarking.heapProfilerDump('renderer', 'final');
           chrome.memoryBenchmarking.heapProfilerDump('browser', 'final');
         }
       """)