webkit_layout_tests failing on chromium.webkit/WebKit Mac10.11 (retina)

Example failure: https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.11%20%28retina%29/builds/19425

Log  https://chromium-swarm.appspot.com/task?id=39fdb38640802410&refresh=10&show_raw=1

Flakiness dashboard https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=%20external%2Fwpt%2FWebCryptoAPI%2FgenerateKey%2Ftest_successes_RSASSA-PKCS1-v1_5.https.html