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

tools/perf/measurements/page_cycler.py

Index: tools/perf/measurements/page_cycler.py
diff --git a/tools/perf/measurements/page_cycler.py b/tools/perf/measurements/page_cycler.py
index bb394b089b3cccf0a550927112ce5a38189b8423..babf272f39b8b21f85ab13c5ac4ab891f07d4759 100644
--- a/tools/perf/measurements/page_cycler.py
+++ b/tools/perf/measurements/page_cycler.py
@@ -39,7 +39,7 @@ class PageCycler(page_measurement.PageMeasurement):
 
     self._speedindex_metric = speedindex.SpeedIndexMetric()
     self._memory_metric = None
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
     self._cpu_metric = None
     self._v8_object_stats_metric = None
     self._has_loaded_page = collections.defaultdict(int)
@@ -81,6 +81,10 @@ class PageCycler(page_measurement.PageMeasurement):
     else:
       cls._cold_run_start_index = args.pageset_repeat * args.page_repeat
 
+  def WillStartBrowser(self, browser):
+    """Initialize metrics once right before the browser has been launched."""
+    self._power_metric = power.PowerMetric(browser)
+
   def DidStartBrowser(self, browser):
     """Initialize metrics once right after the browser has been launched."""
     self._memory_metric = memory.MemoryMetric(browser)