AFAICT this started here: https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests%20%28dbg%29/4277 . Output isn't particularly helpful:

03:44:57.954 75970 worker/1 editing/selection/modify_move/move-by-word-visually-crash-test-5.html output stderr lines:
03:44:57.954 75970   objc[76023]: Class MockCrApp is implemented in both /b/s/w/ir/out/Debug/libtest_runner.dylib (0x13b80ea50) and /b/s/w/ir/out/Debug/Content Shell.app/Contents/Frameworks/Content Shell Framework.framework/Versions/C/Content Shell Framework (0x105534058). One of the two will be used. Which one is undefined.
03:44:57.954 75970   [76015:775:0726/034328.508239:ERROR:render_process_impl.cc(210)] WebFrame LEAKED 1 TIMES
03:44:57.956 74590 [24/24] editing/selection/modify_move/move-by-word-visually-crash-test-5.html failed unexpectedly (test timed out)
03:44:57.955 75970 worker/1 killing primary driver
03:44:57.956 75970 worker/1 killing secondary driver
03:44:57.956 75970 worker/1 editing/selection/modify_move/move-by-word-visually-crash-test-5.html failed:
03:44:57.956 75970 worker/1  test timed out
03:44:57.956 75970 worker/1 killing secondary driver
03:44:57.957 75970 worker/1 exiting

Assuming I have the first failure right, the only blink related patch is https://chromium-review.googlesource.com/c/chromium/src/+/1150010 .