rebaseline_server: add --compare-configs option

This allows us to compare GMs between configs across all builders, so we can see the largest deviations between raster and GPU renderings.

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

Author: epoger@google.com

Review URL: https://codereview.chromium.org/215503002

git-svn-id: http://skia.googlecode.com/svn/trunk@13991 2bbb7eff-a529-9590-31e7-b0007b416f81
8 files changed