Fix format produced by chart data recorder.

mojo/devtools/common/mojo_benchmark

 #!/usr/bin/env python
 # 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.
 
 """Runner for Mojo application benchmarks."""
 
 import argparse
+import json
 import logging
+import os.path
+import re
 import sys
 import time
-import os.path
-import re
 
 from devtoolslib import shell_arguments
 from devtoolslib import shell_config
 from devtoolslib import perf_dashboard
 
 
 _DESCRIPTION = """Runner for Mojo application benchmarks.
 
 |benchmark_list_file| has to be a valid Python program that sets a |benchmarks|
 global variable, containing entries of the following form:
@@ skipping 213 matching lines @@
           print 'benchmark failed: ' + benchmark_error
         if some_measurements_failed:
           print 'some measurements failed'
         print 'output: '
         print '-' * 72
         print output
         print '-' * 72
         exit_code = 1
 
       if script_args.chart_data_output_file:
-        script_args.chart_data_output_file.write(chart_data_recorder.get_json())
+        script_args.chart_data_output_file.write(
+            json.dumps(chart_data_recorder.get_chart_data()))
         script_args.chart_data_output_file.write('\n')
 
   return exit_code
 
 if __name__ == '__main__':
   sys.exit(main())