rebaseline_server: make intermediate JSON specify base urls for diff images

BUG=skia:1919,skia:2230
NOTREECHECKS=True
NOTRY=True
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/196533021

git-svn-id: http://skia.googlecode.com/svn/trunk@13830 2bbb7eff-a529-9590-31e7-b0007b416f81
8 files changed