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

tools/perf/benchmarks/sunspider.py

Index: tools/perf/benchmarks/sunspider.py
diff --git a/tools/perf/benchmarks/sunspider.py b/tools/perf/benchmarks/sunspider.py
index 62a6a50b330b5a31d51d950e0f8be99f74b2fd8f..aef991d80906e600859474e943743756209b9200 100644
--- a/tools/perf/benchmarks/sunspider.py
+++ b/tools/perf/benchmarks/sunspider.py
@@ -17,11 +17,14 @@ _URL = 'http://www.webkit.org/perf/sunspider-1.0.2/sunspider-1.0.2/driver.html'
 class _SunspiderMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_SunspiderMeasurement, 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)