As part of https://bugs.webkit.org/show_bug.cgi?id=163659, we've introduced the test at media/modern-media-controls/start-support/start-support-fullscreen.html and skipped it on iOS. The test doesn't seem to be getting a webkitfullscreenchange event when webkitEnterFullscreen() is called, even though we do enter fullscreen.
<rdar://problem/29055595>
This doesn't actually apply on iOS since fullscreen is not handled by the shadow root controls.