rebaseline.py: if expectations dir contains JSON format results, update those instead of image files

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 27 matching lines @@
   # otherwise).
   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."""
+  with open(file_path, 'w') as outfile:
+    json.dump(json_dict, outfile, sort_keys=True, indent=2)