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

tools/perf/measurements/image_decoding.py

Index: tools/perf/measurements/image_decoding.py
diff --git a/tools/perf/measurements/image_decoding.py b/tools/perf/measurements/image_decoding.py
index ece8f0e9f5768a9d2bef7506ac47c723f155348d..2ec61c786b1cc30c2663dd8ec9a2640d110cf513 100644
--- a/tools/perf/measurements/image_decoding.py
+++ b/tools/perf/measurements/image_decoding.py
@@ -10,12 +10,15 @@ from telemetry.timeline import model
 class ImageDecoding(page_measurement.PageMeasurement):
   def __init__(self):
     super(ImageDecoding, self).__init__()
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
 
   def CustomizeBrowserOptions(self, options):
     options.AppendExtraBrowserArgs('--enable-gpu-benchmarking')
     power.PowerMetric.CustomizeBrowserOptions(options)
 
+  def WillStartBrowser(self, browser):
+    self._power_metric = power.PowerMetric(browser)
+
   def WillNavigateToPage(self, page, tab):
     tab.ExecuteJavaScript("""
         if (window.chrome &&