Reviewed by Kevin Decker

    Reviewed by Kevin Decker

        - fixed <rdar://problem/4184640> "Look Up in Dictionary" item is always disabled for PDF pages

        * WebView/WebPDFView.mm:
        (-[WebPDFView validateUserInterfaceItem:]):
        enable "Look Up in Dictionary" only if we're using a version of PDFKit that knows how to do so
        (-[WebPDFView _canLookUpInDictionary]):
        use respondsToSelector to test whether the current version of PDFKit supports this non-API feature
        (-[WebPDFView _lookUpInDictionaryFromMenu:]):
        implement this method, which WebKit includes in the context menu when there's selected text
        (-[WebPDFView _menuItemsFromPDFKitForEvent:]):
        updated comment for this change

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@20920 268f45cc-cd09-0410-ab3c-d52691b4dbfc