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

tools/perf/benchmarks/kraken.py

Index: tools/perf/benchmarks/kraken.py
diff --git a/tools/perf/benchmarks/kraken.py b/tools/perf/benchmarks/kraken.py
index fa0af6de5a8483becddf6d8111bc0dddba5b9016..a4687c9ae333d3147956e31dc489028272ea8d7c 100644
--- a/tools/perf/benchmarks/kraken.py
+++ b/tools/perf/benchmarks/kraken.py
@@ -19,11 +19,14 @@ def _Mean(l):
 class _KrakenMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_KrakenMeasurement, 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)