Layout test imported/w3c/web-platform-tests/WebCryptoAPI/derive_bits_keys/test_hkdf.https.html started timing out flakily about two days ago. First known timeout was r232869. It was quite reliably until this point. I also have one recorded timeout for imported/w3c/web-platform-tests/WebCryptoAPI/derive_bits_keys/test_pbkdf2_short_long.https.html. I assume it's related.
The test(s) filed under this bug have been consistently passing for the last 4000 revisions. Marking bug as fixed. Committed r263254: <https://trac.webkit.org/changeset/263254>