MediaStream API: Add the mediaStream constructor

MediaStream API: Add the mediaStream constructor
https://bugs.webkit.org/show_bug.cgi?id=76436

Adding support for creating a MediaStream using a collection of MediaStreamTracks.

Reviewed by Adam Barth.

Tests for the Media Stream API will be provided by the bug 56587, pending enough landed code.

* mediastream/MediaStream.cpp:
(WebCore::MediaStream::create):
* mediastream/MediaStream.h:
* mediastream/MediaStream.idl:
* mediastream/MediaStreamTrack.cpp:
(WebCore::MediaStreamTrack::component):
* mediastream/MediaStreamTrack.h:
* platform/mediastream/MediaStreamCenter.cpp:
(WebCore::MediaStreamCenter::didConstructMediaStream):
* platform/mediastream/MediaStreamCenter.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105138 268f45cc-cd09-0410-ab3c-d52691b4dbfc