Add image_diff from the chromium repo.

Add image_diff from the chromium repo.

This is needed for reference tests so we can diff images.
This is just a straight copy of the image diff in the
chromium repo.

R=eseidel@google.com, esprehn@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/ac5097e515ce8237ead2c89bc61dc914cb75313b

[ojan, 2014-12-11 17:41:59]

ojan@chromium.org changed reviewers:
+ eseidel@chromium.org, esprehn@chromium.org, jamesr@chromium.org

[ojan, 2014-12-11 17:42:00]

Wasn't sure whether to put this in sky/tools or tools. I imagine we'll want
image diffing for other parts of the project eventually. I went with this
because it seemed simplest. We can always move it in the future.

If folks feel strongly, I can move it now. I don't have a preference.

[eseidel1, 2014-12-11 17:43:33]

eseidel@google.com changed reviewers:
+ eseidel@google.com

[eseidel1, 2014-12-11 17:43:34]

Can we just deps it in?  Seems a bit silly to copy we're not gonna change the
code :)  otherwise lgtm

[esprehn, 2014-12-11 17:43:44]

lgtm

[ojan, 2014-12-11 17:50:33]

On 2014/12/11 at 17:43:34, eseidel wrote:
> Can we just deps it in?  Seems a bit silly to copy we're not gonna change the
code :)  otherwise lgtm

In chromium it's checked into the actual repo. Not pulled in via DEPS. Is there
a way to deps in part of a repository? I didn't think there was. We could pull
it out of chromium into a dep that both mojo and chromium share, but that
doesn't seem worth it for this tiny little library.

[jamesr, 2014-12-11 18:24:23]

On 2014/12/11 17:50:33, ojan wrote:
> On 2014/12/11 at 17:43:34, eseidel wrote:
> > Can we just deps it in?  Seems a bit silly to copy we're not gonna change
the
> code :)  otherwise lgtm
> 
> In chromium it's checked into the actual repo. Not pulled in via DEPS. Is
there
> a way to deps in part of a repository?

Not really.

> I didn't think there was. We could pull
> it out of chromium into a dep that both mojo and chromium share,

Yeah, and you can create a repo that automatically mirrors a subdirectory but
it's really not worth it.

> but that doesn't seem worth it for this tiny little library.

I agree.  This code pretty much hasn't changed in chromium other than purely
mechanical changes in over a year.

[ojan, 2014-12-11 23:21:33]

Committed patchset #1 (id:1) manually as
ac5097e515ce8237ead2c89bc61dc914cb75313b (presubmit successful).

[blundell, 2014-12-12 07:12:20]

On 2014/12/11 23:21:33, ojan wrote:
> Committed patchset #1 (id:1) manually as
> ac5097e515ce8237ead2c89bc61dc914cb75313b (presubmit successful).

FYI, I revert this CL in https://codereview.chromium.org/800773002/ as it broke
the asan build.