Move skdiff tool and add to gn build.

I really wanted this today, so I got it working again.

Change-Id: I1a37d48d4806198b55c59d1df5ff15a03500195f
Reviewed-on: https://skia-review.googlesource.com/3383
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
10 files changed