rebaseline_server: use new intermediate JSON format

gm/rebaseline_server/imagepair.py

Index: gm/rebaseline_server/imagepair.py
diff --git a/gm/rebaseline_server/imagepair.py b/gm/rebaseline_server/imagepair.py
index bba36fa8c4b4e47ab8101e1bcb580cba6095dee6..a79f3e4ef465fec706925c7fbbe43da66014e788 100644
--- a/gm/rebaseline_server/imagepair.py
+++ b/gm/rebaseline_server/imagepair.py
@@ -12,6 +12,7 @@ ImagePair class (see class docstring for details)
 import posixpath
 
 # Keys used within ImagePair dictionary representations.
+# Keep these in sync with static/constants.js
 KEY__DIFFERENCE_DATA = 'differenceData'
 KEY__EXPECTATIONS_DATA = 'expectations'
 KEY__EXTRA_COLUMN_VALUES = 'extraColumns'
@@ -31,8 +32,10 @@ class ImagePair(object):
     Args:
       image_diff_db: ImageDiffDB instance we use to generate/store image diffs
       base_url: base of all image URLs
-      imageA_relative_url: URL pointing at an image, relative to base_url
-      imageB_relative_url: URL pointing at an image, relative to base_url
+      imageA_relative_url: string; URL pointing at an image, relative to
+          base_url; or None, if this image is missing
+      imageB_relative_url: string; URL pointing at an image, relative to
+          base_url; or None, if this image is missing
       expectations: optional dictionary containing expectations-specific
           metadata (ignore-failure, bug numbers, etc.)
       extra_columns: optional dictionary containing more metadata (test name,
@@ -45,6 +48,10 @@ class ImagePair(object):
     self.extra_columns_dict = extra_columns
     if imageA_relative_url == imageB_relative_url:
       self.diff_record = None
+    elif not imageA_relative_url:
+      self.diff_record = None
+    elif not imageB_relative_url:
+      self.diff_record = None
     else:
       # TODO(epoger): Rather than blocking until image_diff_db can read in
       # the image pair and generate diffs, it would be better to do it