Reviewed by Tony Chang.

        Refactor RenderObject::draggableNode.
        https://bugs.webkit.org/show_bug.cgi?id=60503

        This patch lays the ground for refactoring the drag start logic. It moves draggableNode to
        DragController to avoid the awkward plumbing of calling DragController from RenderObject and
        also moves EventHandlerDragState out from EventHandler so it can be shared between
        EventHandler and DragController where appropriate.

        No new tests since there should be no behavior change.

        * GNUmakefile.list.am:
        * WebCore.gypi:
        * WebCore.pro:
        * WebCore.vcproj/WebCore.vcproj:
        * WebCore.xcodeproj/project.pbxproj:
        * page/DragController.cpp:
        (WebCore::DragController::draggableNode):
        (WebCore::DragController::mayStartDragAtEventLocation):
        * page/DragController.h:
        * page/DragState.h: Added.
        (WebCore::DragState::DragState):
        * page/EventHandler.cpp:
        (WebCore::EventHandler::dragState):
        (WebCore::EventHandler::eventMayStartDrag):
        (WebCore::EventHandler::handleDrag):
        * page/EventHandler.h:
        * rendering/RenderObject.cpp:
        * rendering/RenderObject.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@86128 268f45cc-cd09-0410-ab3c-d52691b4dbfc