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

tools/perf/benchmarks/indexeddb_perf.py

Index: tools/perf/benchmarks/indexeddb_perf.py
diff --git a/tools/perf/benchmarks/indexeddb_perf.py b/tools/perf/benchmarks/indexeddb_perf.py
index cad569b25cf08f245ddfa7f3668cac34fc2dbe2d..ec93f9a50e5e98448dbd0f590c05777669693970 100644
--- a/tools/perf/benchmarks/indexeddb_perf.py
+++ b/tools/perf/benchmarks/indexeddb_perf.py
@@ -38,9 +38,13 @@ 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)