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

gm/rebaseline_server/results_test.py

Index: gm/rebaseline_server/results_test.py
diff --git a/gm/rebaseline_server/results_test.py b/gm/rebaseline_server/results_test.py
index 8263f58920afdf3c5d81f984b89b063bed4062a2..bfb0c8375acc660d3adadc63da1205f9c06e9c01 100755
--- a/gm/rebaseline_server/results_test.py
+++ b/gm/rebaseline_server/results_test.py
@@ -77,6 +77,10 @@ class ResultsTest(unittest.TestCase):
                         os.path.join(self._output_dir_actual, 'gm.json'))
 
 
+# TODO(epoger): create_empty_dir(), find_different_files(), etc. should be
+# extracted from this file to some common location, where they can be shared
+# with download_test.py and other users.
+
 def create_empty_dir(path):
   """Create an empty directory at the given path."""
   if os.path.isdir(path):