rebaseline.py: if expectations dir contains JSON format results, update those instead of image files

Part of Step 3 in https://goto.google.com/ChecksumTransitionDetail

R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/18348018

git-svn-id: http://skia.googlecode.com/svn/trunk@9910 2bbb7eff-a529-9590-31e7-b0007b416f81
13 files changed