teach rebaseline_server how to compare results of multiple render_pictures runs

BUG=skia:2230,skia:1942
NOTRY=True
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/216103004

git-svn-id: http://skia.googlecode.com/svn/trunk@14062 2bbb7eff-a529-9590-31e7-b0007b416f81
12 files changed