The error message is written to stderr on the startup of the WebContent process. It can be found in the stderr logs when running layout tests.
<rdar://problem/39493895>
Created attachment 338128 [details] Patch
Comment on attachment 338128 [details] Patch r=me
Comment on attachment 338128 [details] Patch Thanks for reviewing!
Comment on attachment 338128 [details] Patch Clearing flags on attachment: 338128 Committed r231307: <https://trac.webkit.org/changeset/231307>
All reviewed patches have been landed. Closing bug.