Is this the output that shows image and render tree diffs? If so, I think this is implemented in the old SoM but not in the new one (AFAIK).

Ah sorry, I forgot I was using sheriff-o-matic-staging at the time. Would have been useful to mention.

What I've dug up so far:

This will probably require porting some or all of this logic into alerts-dispatcher:
https://cs.chromium.org/chromium/infra/appengine/sheriff_o_matic/scripts/results.js

It does some regexp munging to construct test result URLs and then does multiple HEAD requests to various https://storage.googleapis.com/chromium-layout-test-archives/... subpaths. On further inspection they don't appear to include build numbers (at least the old SOM client doesn't use any in the urls it fetches). I guess it only looks for latest run? 

Although it does look like the recipe step does include builder/build number somehow in what it uploads:

https://cs.chromium.org/chromium/build/scripts/slave/chromium/archive_layout_test_results.py?sq=package:chromium&dr=C&l=202

So I think what might be happening in the old client is that it probes only for the latest test results in the archives. Either way, the alerts-dispatcher port should look for the results of the proper build number.

I'll start on this today.

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra.git/+/782a19cf14329f0b42c98780680e31bd1fcb3f29

commit 782a19cf14329f0b42c98780680e31bd1fcb3f29
Author: Sean McCullough <seanmccullough@chromium.org>
Date: Wed Aug 24 20:41:51 2016

[som] add webkit test details

BUG= 636252 

Change-Id: Ifecccde255e54861372cb9094773308a1c1a36d5
Reviewed-on: https://chromium-review.googlesource.com/374661
Commit-Queue: Sean McCullough <seanmccullough@chromium.org>
Reviewed-by: Tiffany Zhang <zhangtiff@chromium.org>

[modify] https://crrev.com/782a19cf14329f0b42c98780680e31bd1fcb3f29/go/src/infra/appengine/sheriff-o-matic/README.md
[modify] https://crrev.com/782a19cf14329f0b42c98780680e31bd1fcb3f29/go/src/infra/appengine/sheriff-o-matic/elements/som-examine.html
[add] https://crrev.com/782a19cf14329f0b42c98780680e31bd1fcb3f29/go/src/infra/appengine/sheriff-o-matic/elements/som-webkit-tests.html
[modify] https://crrev.com/782a19cf14329f0b42c98780680e31bd1fcb3f29/go/src/infra/appengine/sheriff-o-matic/test/index.html
[modify] https://crrev.com/782a19cf14329f0b42c98780680e31bd1fcb3f29/go/src/infra/appengine/sheriff-o-matic/test/som-alert-item-test.html
[modify] https://crrev.com/782a19cf14329f0b42c98780680e31bd1fcb3f29/go/src/infra/appengine/sheriff-o-matic/test/som-examine-test.html
[add] https://crrev.com/782a19cf14329f0b42c98780680e31bd1fcb3f29/go/src/infra/appengine/sheriff-o-matic/test/som-webkit-tests-test.html

This should go live in the next push. Note that this code makes guesses about which test output files exist and should be displayed based on some (to me at least) sketchy heuristics. It isn't always correct. Longer term, we should try to be more precise about which output files to expect for a given test failure.

In the mean time, beware that sometimes you'll see "NoSuchKeyThe specified key does not exist." messages where the test actual/expected/diff iframes should be.  That might mean we guessed the wrong file names, but another cause might be that the archive_webkit_tests_results step failed.

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra.git/+/0492c479be537ecbea469ab410da5dfcecd84d9b

commit 0492c479be537ecbea469ab410da5dfcecd84d9b
Author: Sean McCullough <seanmccullough@chromium.org>
Date: Thu Aug 25 01:16:15 2016

[som] Add new link to /results.html for webkit_test results, refactor url formatting.

BUG= 636252 

Change-Id: Idc4521cbbc6ca86a4f01443bd65bee2943125129
Reviewed-on: https://chromium-review.googlesource.com/375518
Reviewed-by: Tiffany Zhang <zhangtiff@chromium.org>
Commit-Queue: Sean McCullough <seanmccullough@chromium.org>

[modify] https://crrev.com/0492c479be537ecbea469ab410da5dfcecd84d9b/go/src/infra/appengine/sheriff-o-matic/elements/som-webkit-tests.html
[add] https://crrev.com/0492c479be537ecbea469ab410da5dfcecd84d9b/go/src/infra/appengine/sheriff-o-matic/elements/urlfmt.js

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra.git/+/0492c479be537ecbea469ab410da5dfcecd84d9b

commit 0492c479be537ecbea469ab410da5dfcecd84d9b
Author: Sean McCullough <seanmccullough@chromium.org>
Date: Thu Aug 25 01:16:15 2016

[som] Add new link to /results.html for webkit_test results, refactor url formatting.

BUG= 636252 

Change-Id: Idc4521cbbc6ca86a4f01443bd65bee2943125129
Reviewed-on: https://chromium-review.googlesource.com/375518
Reviewed-by: Tiffany Zhang <zhangtiff@chromium.org>
Commit-Queue: Sean McCullough <seanmccullough@chromium.org>

[modify] https://crrev.com/0492c479be537ecbea469ab410da5dfcecd84d9b/go/src/infra/appengine/sheriff-o-matic/elements/som-webkit-tests.html
[add] https://crrev.com/0492c479be537ecbea469ab410da5dfcecd84d9b/go/src/infra/appengine/sheriff-o-matic/elements/urlfmt.js