mac: Expose keychain access frequency to Telemetry.

tools/perf/metrics/keychain_metric.py

+# Copyright 2014 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 metrics import Metric
+from telemetry.value import scalar
+
+
+class KeychainMetric(Metric):
+  """KeychainMetric gathers keychain statistics from the browser object.
+
+  This includes the number of times that the keychain was accessed.
+  """
+
+  DISPLAY_NAME = 'OSX_Keychain_Access'
+  HISTOGRAM_NAME = 'OSX.Keychain.Access'
+
+  def AddResults(self, tab, results):
+    """Adds the number of times that the keychain was accessed to |results|."""
+    get_histogram_js = 'statsCollectionController.getBrowserHistogram("%s")'
+
+    result = tab.EvaluateJavaScript(get_histogram_js % self.HISTOGRAM_NAME)
+    result = json.loads(result)
+
+    access_count = 0
+    if result:

[jeremy, 2014/11/13 11:31:30]

Do you think we should fail if there are no entries for this histogram?

[erikchen, 2014/11/13 22:52:59]

If the keychain is never accessed, then there are no entries for the histogram,
right? If that is true, then this would probably apply to most of the telemetry
tests.

+      access_count = result['sum']
+    results.AddValue(scalar.ScalarValue(
+        results.current_page, self.DISPLAY_NAME, 'count', access_count))