They have started after this change:
(Roll src/third_party/skia 9ec77b5574b4..1d6281d4bb47)

https://chromium.googlesource.com/chromium/src/+/8c2d5748bdc2b81bf3e93755939c0093bbd54775

reed@,

Could you please take a look and help in assigning it?

Revert the roll? Or just whatever change broke it? I'm guessing this is due to the switch to readpixels because we're timing out.

Where is the source to the test? If this is timing out, that seems rather fragile (in timing)

Is this part of the failure?

07:04:23.434 11200 worker/5 external/wpt/worklets/animation-worklet-csp.https.html output stderr lines:
07:04:23.434 11200   objc[12543]: Class MockCrApp is implemented in both /b/s/w/ir/out/Debug/libtest_runner.dylib (0x14286b8a0) and /b/s/w/ir/out/Debug/Content Shell.app/Contents/Frameworks/Content Shell Framework.framework/Versions/C/Content Shell Framework (0x10547e418). One of the two will be used. Which one is undefined.
07:04:23.434 11200   [12543:775:0919/070421.221781:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode
07:04:23.434 11200   CONSOLE ERROR: Refused to load the script 'https://www1.web-platform.test:8444/worklets/resources/import-empty-worklet-script-with-cors-header.js' because it violates the following Content Security Policy directive: "script-src 'self' 'unsafe-inline'". Note that 'script-src-elem' was not explicitly set, so 'script-src' is used as a fallback.
07:04:23.434 11200   
07:04:23.434 11200   [12543:775:0919/070421.503790:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode
07:04:23.434 11200   CONSOLE ERROR: Refused to load the script 'https://www1.web-platform.test:8444/worklets/resources/empty-worklet-script.js' because it violates the following Content Security Policy directive: "script-src 'self' 'unsafe-inline'". Note that 'script-src-elem' was not explicitly set, so 'script-src' is used as a fallback.
07:04:23.434 11200   
07:04:23.434 11200   [12543:775:0919/070421.748778:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode
07:04:23.435 11200   [12543:775:0919/070421.980652:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode
07:04:23.435 11200   [12543:775:0919/070422.238583:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode
07:04:23.435 11200   [12543:775:0919/070422.473364:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode
07:04:23.435 11200   [12543:775:0919/070422.688934:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode
07:04:23.435 11200   [12543:775:0919/070422.896966:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode
07:04:23.435 11200   [12543:775:0919/070423.132893:ERROR:layer_tree_host.cc(1001)] LayerTreeMutator not supported in single-thread mode

Where is the c code that this is boiling down to (in chrome)? I'd like to understand where/how chrome is calling into skia to support these tests.

The change in question is a large reduction in complexity/duplication for skia, so before we actually revert it, I want to know if the extra complexity is warranted.

Shall we update the test expectations? That'll clear up the bots, and sheriffs won't bug you while you work on this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/635b955d9fa60a9061e7da95f3651565ff938c81

commit 635b955d9fa60a9061e7da95f3651565ff938c81
Author: Asanka Herath <asanka@chromium.org>
Date: Wed Sep 19 21:09:59 2018

Update expectations for flaky webkit_layout_tests

The following tests are failing on some but not all Linux and Mac
configurations:

* http/tests/csspaint/invalidation-background-image.html
* http/tests/csspaint/invalidation-border-image.html
* http/tests/csspaint/invalidation-content-image.html

Updating expectations while a fix is being worked on.

Bug: 886566
Change-Id: Ic1df9c3d99d189104ee07f873561889c0e192221
Reviewed-on: https://chromium-review.googlesource.com/1234893
Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592547}
[modify] https://crrev.com/635b955d9fa60a9061e7da95f3651565ff938c81/third_party/WebKit/LayoutTests/TestExpectations