teach rebaseline_server to generate diffs of rendered SKPs

Creates a new live-view.html page, served by the rebaseline_server, that will display diffs between two sets of rendered SKP images.

BUG=skia:1942
NOTRY=true
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/424263005
12 files changed