rebaseline.py: try to download images from Google Storage before skia-autogen

This will allow us to complete Step 1 of https://goto.google.com/ChecksumTransitionDetail

R=scroggo@google.com

Review URL: https://codereview.chromium.org/16311011

git-svn-id: http://skia.googlecode.com/svn/trunk@9534 2bbb7eff-a529-9590-31e7-b0007b416f81
8 files changed