rebaseline_server: include setA/setB descriptions in output JSON

gm/gm_json.py

 #!/usr/bin/env python
 # Copyright (c) 2013 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.
 
 """Schema of the JSON summary file written out by the GM tool.
 
 This must be kept in sync with the kJsonKey_ constants in gm_expectations.cpp !
 """
 
@@ skipping 145 matching lines @@
     url: a URL generated with CreateGmRelativeUrl().
 
   Returns: (test_name, hash_type, hash_digest) tuple.
   """
   hash_type, test_name, hash_digest = GM_RELATIVE_URL_RE.match(url).groups()
   return (test_name, hash_type, hash_digest)
 
 
 def LoadFromString(file_contents):
   """Loads the JSON summary written out by the GM tool.
+
      Returns a dictionary keyed by the values listed as JSONKEY_ constants
-     above."""
+     above; if file_contents is empty, returns None."""
   # TODO(epoger): we should add a version number to the JSON file to ensure
   # that the writer and reader agree on the schema (raising an exception
   # otherwise).
+  if not file_contents:
+    return None
   json_dict = json.loads(file_contents)
   return json_dict
 
 
 def LoadFromFile(file_path):
   """Loads the JSON summary written out by the GM tool.
      Returns a dictionary keyed by the values listed as JSONKEY_ constants
      above."""
   file_contents = open(file_path, 'r').read()
   return LoadFromString(file_contents)
 
 
 def WriteToFile(json_dict, file_path):
   """Writes the JSON summary in json_dict out to file_path.
 
   The file is written Unix-style (each line ends with just LF, not CRLF);
   see https://code.google.com/p/skia/issues/detail?id=1815 for reasons."""
   with io.open(file_path, mode='w', newline='', encoding='utf-8') as outfile:
     outfile.write(unicode(json.dumps(json_dict, outfile, sort_keys=True,
                                      indent=2)))