rebaseline_server: use just skpdiff, not Python Image Library

gm/rebaseline_server/imagepairset.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.
 
 ImagePairSet class; see its docstring below.
 """
 
 # System-level imports
 import posixpath
 
 # Local imports
 import column
+import imagediffdb
 import imagepair
 
 # Keys used within dictionary representation of ImagePairSet.
 # NOTE: Keep these in sync with static/constants.js
 KEY__ROOT__EXTRACOLUMNHEADERS = 'extraColumnHeaders'
 KEY__ROOT__HEADER = 'header'
 KEY__ROOT__IMAGEPAIRS = 'imagePairs'
 KEY__ROOT__IMAGESETS = 'imageSets'
 KEY__IMAGESETS__FIELD__BASE_URL = 'baseUrl'
 KEY__IMAGESETS__FIELD__DESCRIPTION = 'description'
@@ skipping 123 matching lines @@
                 key_description: self._descriptions[0],
                 key_base_url: self._image_base_url,
             },
             KEY__IMAGESETS__SET__IMAGE_B: {
                 key_description: self._descriptions[1],
                 key_base_url: self._image_base_url,
             },
             KEY__IMAGESETS__SET__DIFFS: {
                 key_description: 'color difference per channel',
                 key_base_url: posixpath.join(
-                    self._diff_base_url, 'diffs'),
+                    self._diff_base_url, imagediffdb.RGBDIFFS_SUBDIR),
             },
             KEY__IMAGESETS__SET__WHITEDIFFS: {
                 key_description: 'differing pixels in white',
                 key_base_url: posixpath.join(
-                    self._diff_base_url, 'whitediffs'),
+                    self._diff_base_url, imagediffdb.WHITEDIFFS_SUBDIR),
             },
         },
     }