rebaseline_server: cache results in long-running ImageDiffDB instance

gm/rebaseline_server/imagediffdb.py

Index: gm/rebaseline_server/imagediffdb.py
diff --git a/gm/rebaseline_server/imagediffdb.py b/gm/rebaseline_server/imagediffdb.py
index 985017af1c28b862525957588df4e8482ca10541..f6071f9700661abb3d405881e5eba751a2ede7d0 100644
--- a/gm/rebaseline_server/imagediffdb.py
+++ b/gm/rebaseline_server/imagediffdb.py
@@ -175,6 +175,8 @@ class DiffRecord(object):
     finally:
       shutil.rmtree(skpdiff_output_dir)
 
+  # TODO(epoger): Use properties instead of getters throughout.
+  # See http://stackoverflow.com/a/6618176
   def get_num_pixels_differing(self):
     """Returns the absolute number of pixels that differ."""
     return self._num_pixels_differing
@@ -221,6 +223,10 @@ class ImageDiffDB(object):
     # actual_image_locator) tuples.
     self._diff_dict = {}
 
+  @property
+  def storage_root(self):
+    return self._storage_root
+
   def add_image_pair(self,
                      expected_image_url, expected_image_locator,
                      actual_image_url, actual_image_locator):