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

tools/perf/benchmarks/robohornet_pro.py

Index: tools/perf/benchmarks/robohornet_pro.py
diff --git a/tools/perf/benchmarks/robohornet_pro.py b/tools/perf/benchmarks/robohornet_pro.py
index d532e6070909f095fede3b09a42d2726f7a89876..8bf531bf325ebef5dae75300fc199afffac63f28 100644
--- a/tools/perf/benchmarks/robohornet_pro.py
+++ b/tools/perf/benchmarks/robohornet_pro.py
@@ -15,11 +15,14 @@ from telemetry.page import page_set
 class _RobohornetProMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_RobohornetProMeasurement, 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)