AFAICT the first failure is here: https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests%20%28dbg%29/4277 . The output isn't terribly helpful:

03:35:51.649 13621   objc[14513]: Class MockCrApp is implemented in both /b/s/w/ir/out/Debug/libtest_runner.dylib (0x139462a50) and /b/s/w/ir/out/Debug/Content Shell.app/Contents/Frameworks/Content Shell Framework.framework/Versions/C/Content Shell Framework (0x10433a058). One of the two will be used. Which one is undefined.
03:35:51.649 13621   CONSOLE WARNING: Mixed Content: The page at 'https://web-platform.test:8444/fetch/cross-origin-resource-policy/scheme-restriction.https.window.html' was loaded over HTTPS, but requested an insecure image 'http://www1.web-platform.test:8001/fetch/cross-origin-resource-policy/resources/image.py?corp=same-site'. This content should also be served over HTTPS.
03:35:51.650 13621 worker/0 external/wpt/fetch/cross-origin-resource-policy/scheme-restriction.https.window.html passed
03:35:51.650 13621 worker/0 killing secondary driver
03:35:51.697 13624 worker/3 external/wpt/fullscreen/api/element-request-fullscreen-same-manual.html passed
03:35:52.063 13624 worker/3 external/wpt/fullscreen/api/promises-resolve-manual.html passed
03:35:52.312 13627 worker/6 external/wpt/encoding/legacy-mb-japanese/shift_jis/sjis-decode.html?4001-5000 passed
03:35:52.382 13622 killed pid 14341
03:35:52.388 13577 [3271/5972] external/wpt/editing/run/outdent.html failed unexpectedly (test timed out)
03:35:52.387 13622 worker/1 killing primary driver
03:35:52.388 13622 worker/1 killing secondary driver
03:35:52.388 13622 worker/1 external/wpt/editing/run/outdent.html failed:
03:35:52.388 13622 worker/1  test timed out

From https://chromium-swarm.appspot.com/task?id=3eeeefa8595bad10&refresh=10&show_raw=1 . 

The regression range is 

Change-Id: I92073b73cc52ac0bacc7446ce3d1389fef5f6c13
Reviewed-on: https://chromium-review.googlesource.com/1150164
Reviewed-by: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Commit-Queue: Oscar Johansson <oscarj@opera.com>
Cr-Commit-Position: refs/heads/master@{#578246}

to

Change-Id: Ie238160a6b84f80c8a5c25f4035307d4a668c00c
Reviewed-on: https://chromium-review.googlesource.com/1151071
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578239}

My guess is https://chromium-review.googlesource.com/1150010 . I'm going to try and revert it.

Can't revert as there are conflicts. Assigning kojii@ as his patch at https://chromium-review.googlesource.com/1150010 seems to be the most likely.

I believe this is  issue 868148 , which the fix landed an hour or so ago. Build 4314 still has 2 failures, but these are new to 4314, I think these are not related.

Sorry for the trouble.