MediaStream API: Adding the onstatechange callback to PeerConnection

MediaStream API: Adding the onstatechange callback to PeerConnection
https://bugs.webkit.org/show_bug.cgi?id=77954

When readyState changes a callback should be triggered.

Reviewed by Adam Barth.

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

* dom/EventNames.h:
(WebCore):
* mediastream/PeerConnection.cpp:
(WebCore::PeerConnection::changeReadyState):
* mediastream/PeerConnection.h:
(PeerConnection):
(WebCore::PeerConnection::didChangeState):
* mediastream/PeerConnection.idl:
* platform/mediastream/PeerConnectionHandlerClient.h:
(PeerConnectionHandlerClient):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107054 268f45cc-cd09-0410-ab3c-d52691b4dbfc