add tool_util to compare images

Bug: skia:
Change-Id: I826190aa15cf42a3ca7f3c7d5271ead5b514c116
Reviewed-on: https://skia-review.googlesource.com/32220
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed