According to the flakiness Dashboard fullscreen/full-screen-iframe-legacy.html is a flaky failure on Sierra WK1. This doesn't appear to be a recent regression.
The test is also expected to either pass or crash on Debug builds currently
@@ -1,3 +1,4 @@
+CONSOLE MESSAGE: line 6388: NotFoundError: The object can not be found here.
Test for bug 61461: Handle entering full screen security restrictions
To test manually, click the video "full screen" button - the page should enter full screen mode.
Marked test as flaky on Sierra WK1 in https://trac.webkit.org/changeset/220820/webkit