Add a stat to the smoothness benchmark for avg number of missing tiles.

tools/perf/perf_tools/smoothness_benchmark.py

 # Copyright (c) 2012 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.
 from telemetry import multi_page_benchmark
 from telemetry import util
 
 class DidNotScrollException(multi_page_benchmark.MeasurementFailure):
   def __init__(self):
     super(DidNotScrollException, self).__init__('Page did not scroll')
 
@@ skipping 19 matching lines @@
     'numMainThreadScrolls', 0)
 
   percent_impl_scrolled = DivideIfPossibleOrZero(
     float(num_impl_thread_scrolls),
     num_impl_thread_scrolls + num_main_thread_scrolls)
 
   num_layers = (
       rendering_stats_deltas.get('numLayersDrawn', 0) /
       float(num_frames_sent_to_screen))
 
+  num_missing_tiles = (
+      rendering_stats_deltas.get('numMissingTiles', 0) /
+      float(num_frames_sent_to_screen))
+
   results.Add('mean_frame_time', 'ms', round(mean_frame_time_seconds * 1000, 3))
   results.Add('dropped_percent', '%', round(dropped_percent * 100, 1),
               data_type='unimportant')
   results.Add('percent_impl_scrolled', '%',
               round(percent_impl_scrolled * 100, 1),
               data_type='unimportant')
   results.Add('average_num_layers_drawn', '', round(num_layers, 1),
               data_type='unimportant')
+  results.Add('average_num_missing_tiles', '', round(num_missing_tiles, 1),
+              data_type='unimportant')
 
 def CalcPaintingResults(rendering_stats_deltas, results):
   totalPaintTime = rendering_stats_deltas.get('totalPaintTimeInSeconds', 0)
   totalRasterizeTime = rendering_stats_deltas.get(
     'totalRasterizeTimeInSeconds', 0)
   totalPixelsPainted = rendering_stats_deltas.get('totalPixelsPainted', 0)
   totalPixelsRasterized = rendering_stats_deltas.get('totalPixelsRasterized', 0)
 
   megapixelsPaintedPerSecond = DivideIfPossibleOrZero(
       (totalPixelsPainted / 1000000.0), totalPaintTime)
@@ skipping 76 matching lines @@
       raise DidNotScrollException()
 
     CalcFirstPaintTimeResults(results, tab)
     CalcScrollResults(rendering_stats_deltas, results)
     CalcPaintingResults(rendering_stats_deltas, results)
     CalcTextureUploadResults(rendering_stats_deltas, results)
 
     if self.options.report_all_results:
       for k, v in rendering_stats_deltas.iteritems():
         results.Add(k, '', v)