[MSE] [Mac] Disclaim support for MSE in AVFoundation and QTKit engines

[MSE] [Mac] Disclaim support for MSE in AVFoundation and QTKit engines
https://bugs.webkit.org/show_bug.cgi?id=123593

Reviewed by Eric Carlson.

Immediately fail if asked to load a Media Source in engines which do not support
them.

* platform/graphics/avfoundation/MediaPlayerPrivateAVFoundation.cpp:
(WebCore::MediaPlayerPrivateAVFoundation::load):
* platform/graphics/avfoundation/MediaPlayerPrivateAVFoundation.h:
* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.mm:
(WebCore::MediaPlayerPrivateAVFoundationObjC::supportsType):
* platform/graphics/mac/MediaPlayerPrivateQTKit.h:
* platform/graphics/mac/MediaPlayerPrivateQTKit.mm:
(WebCore::MediaPlayerPrivateQTKit::load):
(WebCore::MediaPlayerPrivateQTKit::supportsType):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@158597 268f45cc-cd09-0410-ab3c-d52691b4dbfc