[telemetry] Add new measurement that counts number of GCs needed to free V8 context.

tools/perf/measurements/v8_detached_context_age_in_gc.py

Index: tools/perf/measurements/v8_detached_context_age_in_gc.py
diff --git a/tools/perf/measurements/v8_detached_context_age_in_gc.py b/tools/perf/measurements/v8_detached_context_age_in_gc.py
new file mode 100644
index 0000000000000000000000000000000000000000..51dec330727b1c606fd5879ccab40a1a1e4ad7f5
--- /dev/null
+++ b/tools/perf/measurements/v8_detached_context_age_in_gc.py
@@ -0,0 +1,54 @@
+# Copyright 2015 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 json
+from telemetry.page import page_test
+from telemetry.value import histogram
+from telemetry.value import histogram_util
+from telemetry.value import scalar
+from telemetry.value import skip
+
+_NAME = 'V8.DetachedContextAgeInGC'
+_UNITS = 'garbage_collections'
+_DISPLAY_NAME = 'V8_DetachedContextAgeInGC'
+_TYPE = histogram_util.RENDERER_HISTOGRAM
+_DESCRIPTION = 'Number of GCs needed to collect detached context'
+
+
+def _GetMaxDetachedContextAge(tab, data_start):
+  data = histogram_util.GetHistogram(_TYPE, _NAME, tab)
+  delta = histogram_util.SubtractHistogram(data, data_start)
+  if not 'buckets' in delta:
+    return
+  buckets = json.loads(delta)['buckets']
+  if buckets:
+    return max(x.get('high', x['low']) for x in buckets)
+
+
+class V8DetachedContextAgeInGC(page_test.PageTest):
+  def __init__(self):
+    super(V8DetachedContextAgeInGC, self).__init__('RunPageInteractions')
+    self._data_start = None
+
+  def CustomizeBrowserOptions(self, options):
+    options.AppendExtraBrowserArgs(['--enable-stats-collection-bindings'])
+
+  def DidNavigateToPage(self, page, tab):
+    self._data_start = histogram_util.GetHistogram(_TYPE, _NAME, tab)
+
+  def ValidateAndMeasurePage(self, page, tab, results):
+    # Trigger GC to get histogram data.
+    # Seven GCs should be enough to collect any detached context.
+    # If a detached context survives more GCs then there is a leak.
+    MAX_AGE = 8
+    for _ in xrange(MAX_AGE):
+      tab.CollectGarbage()
+    value = _GetMaxDetachedContextAge(tab, self._data_start)
+    if value is None:
+      results.AddValue(skip.SkipValue(
+          results.current_page, 'No detached contexts'))
+    else:
+      results.AddValue(scalar.ScalarValue(
+          results.current_page, _DISPLAY_NAME, _UNITS, value,
+          description=_DESCRIPTION))