rebaseline_server: create ImagePairSet-- holds a number of ImagePairs to examine

See https://goto.google.com/ChangingRbsJson and bug 1919 for additional context

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

Author: epoger@google.com

Review URL: https://codereview.chromium.org/139343018

git-svn-id: http://skia.googlecode.com/svn/trunk@13434 2bbb7eff-a529-9590-31e7-b0007b416f81
5 files changed