sandwich: Make metrics extraction more customizable.

tools/android/loading/sandwich_metrics_unittest.py

 # Copyright 2016 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.
 
 import copy
 import json
 import os
 import shutil
 import subprocess
 import tempfile
@@ skipping 214 matching lines @@
       return puller.CompletenessPoint(time=time,
                                       frame_completeness=frame_completeness)
     completness_record = [
       point(0, 0.0),
       point(120, 0.4),
       point(190, 0.75),
       point(280, 1.0),
       point(400, 1.0),
     ]
     self.assertEqual(120 + 70 * 0.6 + 90 * 0.25,
-                     puller.ComputeSpeedIndex(completness_record))
+                     puller._ComputeSpeedIndex(completness_record))
 
     completness_record = [
       point(70, 0.0),
       point(150, 0.3),
       point(210, 0.6),
       point(220, 0.9),
       point(240, 1.0),
     ]
     self.assertEqual(80 + 60 * 0.7 + 10 * 0.4 + 20 * 0.1,
-                     puller.ComputeSpeedIndex(completness_record))
+                     puller._ComputeSpeedIndex(completness_record))
 
     completness_record = [
       point(90, 0.0),
       point(200, 0.6),
       point(150, 0.3),
       point(230, 1.0),
     ]
     with self.assertRaises(ValueError):
-      puller.ComputeSpeedIndex(completness_record)
+      puller._ComputeSpeedIndex(completness_record)
 
 
 if __name__ == '__main__':
   unittest.main()