Fix the WebKit2 build.

        * WebProcess/WebPage/mac/LayerBackedDrawingAreaMac.mm:
        (WebKit::LayerBackedDrawingArea::syncCompositingLayers):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76197 268f45cc-cd09-0410-ab3c-d52691b4dbfc