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

tools/perf/measurements/memory.py

Index: tools/perf/measurements/memory.py
diff --git a/tools/perf/measurements/memory.py b/tools/perf/measurements/memory.py
index cd40199c15bdb9a247acff3305adabd54379a434..ec745cd62bd40f284686eb40552fb0d39bc5300c 100644
--- a/tools/perf/measurements/memory.py
+++ b/tools/perf/measurements/memory.py
@@ -10,7 +10,10 @@ class Memory(page_measurement.PageMeasurement):
   def __init__(self):
     super(Memory, self).__init__('RunStressMemory')
     self._memory_metric = None
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
+
+  def WillStartBrowser(self, browser):
+    self._power_metric = power.PowerMetric(browser)
 
   def DidStartBrowser(self, browser):
     self._memory_metric = memory.MemoryMetric(browser)