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

tools/perf/benchmarks/octane.py

Index: tools/perf/benchmarks/octane.py
diff --git a/tools/perf/benchmarks/octane.py b/tools/perf/benchmarks/octane.py
index dca1bcec0aeeebf899753cbc3e5c7d50afe4d4d3..c03325222359d72f6c5ffe8660f1241a0917bf0e 100644
--- a/tools/perf/benchmarks/octane.py
+++ b/tools/perf/benchmarks/octane.py
@@ -25,11 +25,13 @@ _GB = 1024 * 1024 * 1024
 class _OctaneMeasurement(page_measurement.PageMeasurement):
   def __init__(self):
     super(_OctaneMeasurement, 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 WillNavigateToPage(self, page, tab):
     if tab.browser.memory_stats['SystemTotalPhysicalMemory'] < 1 * _GB: