rebaseline_server: if creating brand new expectations files, include the header

gm/rebaseline_server/compare_rendered_pictures.py

 #!/usr/bin/python
 
 """
 Copyright 2014 Google Inc.
 
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 Compare results of two render_pictures runs.
 
@@ skipping 333 matching lines @@
               column_ids_in_order=ORDERED_COLUMN_IDS),
       }
 
   def _validate_dict_version(self, result_dict):
     """Raises Exception if the dict is not the type/version we know how to read.
 
     Args:
       result_dict: dictionary holding output of render_pictures; if None,
           this method will return without raising an Exception
     """
+    # TODO(stephana): These values should be defined as constants somewhere,
+    # to be kept in sync between this file and writable_expectations.py
     expected_header_type = 'ChecksummedImages'
     expected_header_revision = 1
 
     if result_dict == None:
       return
     header = result_dict[gm_json.JSONKEY_HEADER]
     header_type = header[gm_json.JSONKEY_HEADER_TYPE]
     if header_type != expected_header_type:
       raise Exception('expected header_type "%s", but got "%s"' % (
           expected_header_type, header_type))
@@ skipping 113 matching lines @@
       source_dir: path to source dir (GS URL, local filepath, or a special
           "repo:" URL type that points at a file within our Skia checkout;
           only the "repo:" URL type will have a commit hash.
     """
     if source_dir.lower().startswith(REPO_URL_PREFIX):
       repo_dir = os.path.join(REPO_BASEPATH, source_dir[len(REPO_URL_PREFIX):])
       return subprocess.check_output(
           args=[git_utils.GIT, 'rev-parse', 'HEAD'], cwd=repo_dir).strip()
     else:
       return None