download_actuals.py: download JSON files from Google Storage instead of skia-autogen

uses google-api-python-client instead of gsutil binary to interact with Google Storage

BUG=skia:553
R=rmistry@google.com, bensong@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/309653005
diff --git a/gm/rebaseline_server/server.py b/gm/rebaseline_server/server.py
index 40874d6..0680779 100755
--- a/gm/rebaseline_server/server.py
+++ b/gm/rebaseline_server/server.py
@@ -250,6 +250,8 @@
             PARENT_DIRECTORY, STATIC_CONTENTS_SUBDIR, GENERATED_HTML_SUBDIR,
             "index.html"),
         config_pairs=config_pairs)
+    # TODO(epoger): Create shareable functions within download_actuals.py that
+    # we can use both there and here to download the actual image results.
     if actuals_repo_url:
       self._actuals_repo = _create_svn_checkout(
           dir_path=actuals_dir, repo_url=actuals_repo_url)