Update documentation in metrics/__init__.py

tools/perf/metrics/__init__.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 class Metric(object):
   """Base class for all the metrics that are used by telemetry measurements.
 
   The Metric class represents a way of measuring something. Metrics are
   helper classes used by PageMeasurements. Each PageMeasurement may use
   multiple metrics; each metric should be focussed on collecting data
@@ skipping 18 matching lines @@
 
   def Stop(self, page, tab):
     """Stop collecting data for this metric (if applicable)."""
     pass
 
   def AddResults(self, tab, results):
     """Add the data collected into the results object for a measurement.
 
     Metrics may implement AddResults to provide a common way to add results
     to the PageMeasurementResults in PageMeasurement.AddMeasurement --
-    results should be added with results.Add(trace_name, unit, value).
+    results should be added with results.AddValue(...).
     """
     raise NotImplementedError()