Intent to unship: HTMLMediaElement.seekToNextFrame

[Paul Adenot]

I intend to unship HTMLMediaElement.seekToNextFrame, that has been shipped for a long time, but never standardized, in https://bugzilla.mozilla.org/show_bug.cgi?id=1336404.

It was never implemented except in Gecko, and Web Codecs (standard, and shipping in Chromium, shipping soon in Gecko and being implemented in WebKit) allows implementing it in a way that's vastly superior.

It will first be disabled via pref, and then removed altogether some time later, or moved to test-only if we can't figure out a way to migrate the half a dozen test that still use it.

Best,

Paul.