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

Some power measurement backends can only measure global power draw for the whole system and not per-process. Try to get a more exact power measurement by measuring the power draw without the browser running and then subtracting that number from the measurement in the test to try to get a more exact number for how much additional power is being used by the test case.

BUG=339180

Review URL: https://codereview.chromium.org/350763005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@280994 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 5 insertions, 1 deletions

diff --git a/tools/perf/benchmarks/indexeddb_perf.py b/tools/perf/benchmarks/indexeddb_perf.py
index cad569b..ec93f9a 100644
--- a/tools/perf/benchmarks/indexeddb_perf.py
+++ b/tools/perf/benchmarks/indexeddb_perf.py
@@ -38,9 +38,13 @@ class _IndexedDbMeasurement(page_measurement.PageMeasurement):
   def __init__(self, *args, **kwargs):
     super(_IndexedDbMeasurement, self).__init__(*args, **kwargs)
     self._memory_metric = None
-    self._power_metric = power.PowerMetric()
+    self._power_metric = None
     self._v8_object_stats_metric = None
 
+  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)