Reviewed by Ada, Lou and Steve.

        Moving basic components of ClipboardMac to Clipboard, 
        last of the basic common code merge will come later (namely setting the 
        drag element/image).
        
        Tidying up in DragController including a few bad PassRefPtr uses and
        removing unnecessary accessors.  Also initialise fields properly to
        fix <rdar:/problems/5014889>

        * dom/Clipboard.cpp:
        (WebCore::Clipboard::Clipboard):
          New constructor
          
        * dom/Clipboard.h:
        (WebCore::Clipboard::isForDragging):
        (WebCore::Clipboard::dragLocation):
        (WebCore::Clipboard::dragImage):
        (WebCore::Clipboard::dragImageElement):
          Exposing common data
          
        * page/DragController.cpp:
        (WebCore::DragController::DragController):
          Initialise m_isHandlingDrag (oops)
        (WebCore::documentFragmentFromDragData):
        (WebCore::DragController::tryDocumentDrag):
          Fixing PassRefPtr misuse
        (WebCore::DragController::canProcessDrag):
        (WebCore::DragController::startDrag):
        (WebCore::DragController::doSystemDrag):
          Make sure we're using the correct coordinate space
          
        * page/EventHandler.cpp:
        (WebCore::EventHandler::handleDrag):
          Reset drag source correctly
          
        * platform/mac/ClipboardMac.h:
        * platform/mac/ClipboardMac.mm:
        (WebCore::ClipboardMac::ClipboardMac):
          update constructor to use new Clipboard constructor



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@19808 268f45cc-cd09-0410-ab3c-d52691b4dbfc