This happens consistently on https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Precise%20Leak/ since https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Precise%20Leak/builds/1598 (stdio attached).

The relevant lines are:

imported/wpt/html/webappapis/scripting/events/onerroreventhandler.html failed:
 leak detected: ({"numberOfLiveActiveDOMObjects":[2,3],"numberOfLiveDocuments":[1,2],"numberOfLiveNodes":[4,12]})

vabr@ was not able to run this locally, because vabr@ could not find instructions on how to enable leak detection (perhaps we could add those to https://chromium.googlesource.com/chromium/src/+/master/docs/testing/layout_tests.md ?).