teach rebaseline_server to generate diffs of rendered SKPs

gm/rebaseline_server/imagepairset.py

Index: gm/rebaseline_server/imagepairset.py
diff --git a/gm/rebaseline_server/imagepairset.py b/gm/rebaseline_server/imagepairset.py
index ef9acbc4bcc1c10fbe46fe9a926c2d8e35c6cf1e..998c280684fc380c78f35c0fc0c9600e1355ef20 100644
--- a/gm/rebaseline_server/imagepairset.py
+++ b/gm/rebaseline_server/imagepairset.py
@@ -12,9 +12,13 @@ ImagePairSet class; see its docstring below.
 # System-level imports
 import posixpath
 
-# Local imports
+# Must fix up PYTHONPATH before importing from within Skia
+import fix_pythonpath  # pylint: disable=W0611
+
+# Imports from within Skia
 import column
 import imagediffdb
+from py.utils import gs_utils
 
 # Keys used within dictionary representation of ImagePairSet.
 # NOTE: Keep these in sync with static/constants.js
@@ -162,6 +166,13 @@ class ImagePairSet(object):
 
     key_description = KEY__IMAGESETS__FIELD__DESCRIPTION
     key_base_url = KEY__IMAGESETS__FIELD__BASE_URL
+    # EPOGER: instead of this special handling for GS URLs vs HTTP URLs, maybe come up with an object than can represent either one?  (or even a local file)
+    if gs_utils.GSUtils.is_gs_url(self._image_base_url):
+      value_base_url = imagediffdb.GSObject(
+          gs_url=self._image_base_url).as_http_url()
+    else:
+      value_base_url = self._image_base_url
+
     return {
         KEY__ROOT__EXTRACOLUMNHEADERS: self._column_headers_as_dict(),
         KEY__ROOT__EXTRACOLUMNORDER: column_ids_in_order,
@@ -169,11 +180,11 @@ class ImagePairSet(object):
         KEY__ROOT__IMAGESETS: {
             KEY__IMAGESETS__SET__IMAGE_A: {
                 key_description: self._descriptions[0],
-                key_base_url: self._image_base_url,
+                key_base_url: value_base_url,
             },
             KEY__IMAGESETS__SET__IMAGE_B: {
                 key_description: self._descriptions[1],
-                key_base_url: self._image_base_url,
+                key_base_url: value_base_url,
             },
             KEY__IMAGESETS__SET__DIFFS: {
                 key_description: 'color difference per channel',