[Mac] Remove call to ClipboardMac::create from EventHandler::createDraggingClipboard
https://bugs.webkit.org/show_bug.cgi?id=116161

Reviewed by Andreas Kling.

* dom/Clipboard.h: Added createForDragAndDrop function.

* page/mac/EventHandlerMac.mm:
(WebCore::EventHandler::createDraggingClipboard): Changed to use new Clipboard function
instead of ClipboardMac::create.

* platform/mac/ClipboardMac.mm:
(WebCore::Clipboard::createForDragAndDrop): Added. For now it does exactly what the
EventHandler::createDraggingClipboard function did, soon to be refactored better.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150124 268f45cc-cd09-0410-ab3c-d52691b4dbfc