[MediaStream] new load method for MediaStreamPrivate objects

[MediaStream] new load method for MediaStreamPrivate objects
https://bugs.webkit.org/show_bug.cgi?id=142346

Reviewed by Eric Carlson.

No new tests, existing mediastream tests cover this use-case.

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::loadResource): Use new load method for MediaStreams.
* platform/graphics/MediaPlayer.cpp:
(WebCore::NullMediaPlayerPrivate::load): New stub.
(WebCore::MediaPlayer::load): New load method for
MediaStreamPrivate.
(WebCore::MediaPlayer::nextBestMediaEngine): Set a new boolean on
the MediaEngineSupportParameters to differenciate with MSE and
normal video resources.
(WebCore::MediaPlayer::loadWithNextMediaEngine): Defer loading of
the MediaStream to the MediaPlayerPrivate backend.
* platform/graphics/MediaPlayer.h:
(WebCore::MediaEngineSupportParameters::MediaEngineSupportParameters):
* platform/graphics/MediaPlayerPrivate.h:
* platform/graphics/avfoundation/MediaPlayerPrivateAVFoundation.h:
(WebCore::MediaPlayerPrivateAVFoundation::load): New empty load
method for MediaStreamPrivate objects.
* platform/graphics/avfoundation/objc/MediaPlayerPrivateMediaSourceAVFObjC.h:
(WebCore::MediaPlayerPrivateMediaSourceAVFObjC::load): Ditto.
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::MediaPlayerPrivateGStreamer::load): Ditto.
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.h:
* platform/graphics/mac/MediaPlayerPrivateQTKit.h:
(WebCore::MediaPlayerPrivateQTKit::load): Ditto.
* platform/mock/mediasource/MockMediaPlayerMediaSource.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@181153 268f45cc-cd09-0410-ab3c-d52691b4dbfc