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

tools/perf/benchmarks/dromaeo.py

Index: tools/perf/benchmarks/dromaeo.py
diff --git a/tools/perf/benchmarks/dromaeo.py b/tools/perf/benchmarks/dromaeo.py
index 692ca3475c13a65860957cfebb012cb1be1e53e2..049565d85b5def5a0674f7bfbbb63922f919a62a 100644
--- a/tools/perf/benchmarks/dromaeo.py
+++ b/tools/perf/benchmarks/dromaeo.py
@@ -14,11 +14,14 @@ from telemetry.page import page_set
 class _DromaeoMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_DromaeoMeasurement, self).__init__()
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
 
   def CustomizeBrowserOptions(self, options):
     power.PowerMetric.CustomizeBrowserOptions(options)
 
+  def WillStartBrowser(self, browser):
+    self._power_metric = power.PowerMetric(browser)
+
   def DidNavigateToPage(self, page, tab):
     self._power_metric.Start(page, tab)