new tool: download all GM images for a given builder, ready for skdiff

gm/gm_json.py

Index: gm/gm_json.py
diff --git a/gm/gm_json.py b/gm/gm_json.py
index 7cd4455f797371acf14c9fe05f576935f4b137da..1aac6632f5674b0adec9f7c915c82d2837ec3308 100644
--- a/gm/gm_json.py
+++ b/gm/gm_json.py
@@ -104,6 +104,8 @@ def CreateGmActualUrl(test_name, hash_type, hash_digest,
   hash_digest: the hash digest of the image to retrieve
   gm_actuals_root_url: root url where actual images are stored
   """
+  # TODO(epoger): Maybe use url_or_path.join() so that, for testing, this can
+  # return either a URL or a local filepath?
   return '%s/%s/%s/%s.png' % (gm_actuals_root_url, hash_type, test_name,
                               hash_digest)