[Telemetry] Add support for capturing V8 object stats to Telemetry.

tools/perf/metrics/v8_object_stats.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.
+
+from metrics import Metric
+from metrics import stats_table_util
+
+
+class V8ObjectStatsMetric(Metric):
+  """V8ObjectStatsMetric gathers statistics on the size of types in the V8 heap.
+
+  It does this by enabling the --track_gc_object_stats flag on V8 and reading
+  these statistics from the StatsTableMetric.
+  """
+
+  def __init__(self):
+    super(V8ObjectStatsMetric, self).__init__()
+    self._stats_table = None
+    self._stats_table = None
+
+  @classmethod
+  def CustomizeBrowserOptions(cls, options):
+    options.AppendExtraBrowserArg('--enable-stats-table')
+    options.AppendExtraBrowserArg(
+        '--js-flags=--track_gc_object_stats --expose_gc')
+
+  def Start(self, page, tab):
+    """Do Nothing."""
+
+  def Stop(self, page, tab):
+    """Get the values in the stats table after the page is loaded."""
+     # Trigger GC to ensure stats are checkpointed.
+    tab.EvaluateJavaScript('window.gc()')
+    self._stats_table = stats_table_util.GetStatsTable(tab)
+
+  def AddResults(self, tab, results):
+    """Add results for this page to the results object."""
+    assert self._stats_table, 'Must call Stop() first'
+    for stat_name in self._stats_table:
+      if ("V8:OsMemoryAllocated" in stat_name or
+          "V8:SizeOf_" in stat_name or
+          "V8:Memory" in stat_name):
+        results.Add(stat_name, 'kb', self._stats_table[stat_name] / 1024.0)