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

tools/perf/benchmarks/indexeddb_perf.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.
 
 """Runs Chromium's IndexedDB performance test. These test:
 
 Databases:
   create/delete
 Keys:
   create/delete
@@ skipping 20 matching lines @@
 from telemetry.page import page_set
 
 _V8_COUNTER_NAMES = [
     'V8.OsMemoryAllocated',
   ]
 
 class _IndexedDbMeasurement(page_measurement.PageMeasurement):
   def __init__(self, *args, **kwargs):
     super(_IndexedDbMeasurement, self).__init__(*args, **kwargs)
     self._memory_metric = None
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
     self._v8_object_stats_metric = None
 
+  def WillStartBrowser(self, browser):
+    """Initialize metrics once right before the browser has been launched."""
+    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._v8_object_stats_metric = (
       v8_object_stats.V8ObjectStatsMetric(_V8_COUNTER_NAMES))
 
   def DidNavigateToPage(self, page, tab):
     self._memory_metric.Start(page, tab)
     self._power_metric.Start(page, tab)
     self._v8_object_stats_metric.Start(page, tab)
@@ skipping 30 matching lines @@
 class IndexedDb(benchmark.Benchmark):
   """Chromium's IndexedDB Performance tests."""
   test = _IndexedDbMeasurement
 
   def CreatePageSet(self, options):
     indexeddb_dir = os.path.join(util.GetChromiumSrcDir(), 'chrome', 'test',
                                  'data', 'indexeddb')
     ps = page_set.PageSet(file_path=indexeddb_dir)
     ps.AddPageWithDefaultRunNavigate('file://perf_test.html')
     return ps