Replace ValueSetOutputFormatter with HistogramSetJsonOutputFormatter.

telemetry/telemetry/internal/results/histogram_set_json_output_formatter.py

Index: telemetry/telemetry/internal/results/histogram_set_json_output_formatter.py
diff --git a/telemetry/telemetry/internal/results/histogram_set_json_output_formatter.py b/telemetry/telemetry/internal/results/histogram_set_json_output_formatter.py
new file mode 100644
index 0000000000000000000000000000000000000000..6c6f397b498dc71dfcbd674834a94e1aed511b70
--- /dev/null
+++ b/telemetry/telemetry/internal/results/histogram_set_json_output_formatter.py
@@ -0,0 +1,28 @@
+# Copyright 2016 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
+import logging
+
+from telemetry.internal.results import output_formatter
+
+
+class HistogramSetJsonOutputFormatter(output_formatter.OutputFormatter):
+  def __init__(self, output_stream, metadata, reset_results):
+    super(HistogramSetJsonOutputFormatter, self).__init__(output_stream)
+    self._metadata = metadata
+    self._reset_results = reset_results
+
+  def Format(self, page_test_results):
+    histograms = page_test_results.AsHistogramDicts(self._metadata)
+    self._output_stream.seek(0)
+    if not self._reset_results:
+      existing = self._output_stream.read()
+      self._output_stream.seek(0)
+      if existing:
+        try:
+          histograms += json.loads(existing)
+        except ValueError:
+          logging.warn('Found existing histograms json but failed to parse it.')
+    json.dump(histograms, self._output_stream)