mojo_benchmark: improve recording the chart data for 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):
+  def __init__(self, benchmark_name):
     self.charts = defaultdict(list)
+    self.benchmark_name = benchmark_name
 
   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)
+    """Returns the JSON string representing the recorded chart data, wrapping
+    it with the required meta data."""
+    chart_data = {
+        'format_version': '1.0',
+        'benchmark_name': self.benchmark_name,
+        'charts': self.charts
+    }
+    return json.dumps(chart_data)