Teach mojo_benchmark to produce chart_data for the perf dashboard.

mojo/devtools/common/devtoolslib/perf_dashboard.py

+# 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.
+
+# Disable the line-too-long warning.
+# pylint: disable=C0301
+"""This module implements the Chromium Performance Dashboard JSON v1.0 data
+format.
+
+See http://www.chromium.org/developers/speed-infra/performance-dashboard/sending-data-to-the-performance-dashboard.
+"""
+
+import json
+from collections import defaultdict
+
+class ChartDataRecorder(object):
+  """Allows one to record measurement values one by one and then generate the
+  JSON string that represents them in the 'chart_data' format expected by the
+  performance dashboard.
+  """
+
+  def __init__(self):
+    self.charts = defaultdict(list)
+
+  def record_scalar(self, chart_name, value_name, units, value):
+    """Records a single measurement value of a scalar type."""
+    self.charts[chart_name].append({
+        'type': 'scalar',
+        'name': value_name,
+        'units': units,
+        'value': value})
+
+  def get_json(self):
+    """Returns the JSON string representing the recorded chart data."""
+    return json.dumps(self.charts)