[Telemetry] Fix handling for two-part per-page value names in Chart JSON.

tools/telemetry/telemetry/results/chart_json_output_formatter.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 collections
 import itertools
 import json
 
 from telemetry.results import output_formatter
 from telemetry.value import summary as summary_module
@@ skipping 20 matching lines @@
     A Chart JSON dict corresponding to the given data.
   """
   summary = summary_module.Summary(page_specific_values)
   values = itertools.chain(
       summary.interleaved_computed_per_page_values_and_summaries,
       summary_values)
   charts = collections.defaultdict(dict)
 
   for value in values:
     if value.page:
-      chart_name, trace_name = (
-          value.GetChartAndTraceNameForPerPageResult())
+      chart_name, trace_name = (value.GetChartAndTraceNameForPerPageResult())
     else:
       chart_name, trace_name = (
           value.GetChartAndTraceNameForComputedSummaryResult(None))
       if chart_name == trace_name:
         trace_name = 'summary'
 
-    assert trace_name not in charts[chart_name]
-
+    # This intentionally overwrites the trace if it already exists because this
+    # is expected of output from the buildbots currently.
+    # See: crbug.com/413393
     charts[chart_name][trace_name] = value.AsDict()
 
   result_dict = {
     'format_version': '0.1',
     'benchmark_name': benchmark_metadata.name,
     'charts': charts
   }
 
   return result_dict
 
 # TODO(eakuefner): Transition this to translate Telemetry JSON.
 class ChartJsonOutputFormatter(output_formatter.OutputFormatter):
   def __init__(self, output_stream, benchmark_metadata):
     super(ChartJsonOutputFormatter, self).__init__(output_stream)
     self._benchmark_metadata = benchmark_metadata
 
   def Format(self, page_test_results):
     json.dump(_ResultsAsChartDict(
         self._benchmark_metadata,
         page_test_results.all_page_specific_values,
         page_test_results.all_summary_values),
               self.output_stream)
     self.output_stream.write('\n')