Reviewed by Simon Fraser.

        WK2 - Multiple crashes in PlatformCALayer::replaceSublayer
        https://bugs.webkit.org/show_bug.cgi?id=52695

        Added a hostingLayer as the parent of the existing drawingLayer.
        The hostingLayer is now the root which is passed to the 
        remote context. It never changes except to track the size
        of the window. The backingLayer is now a child of the 
        hostingLayer, which allow it to switch between tiled and
        non-tiled.

        I also now give back accurate settings for debug borders and
        repaint counters.

        * WebProcess/WebPage/LayerBackedDrawingArea.cpp:
        (WebKit::LayerBackedDrawingArea::LayerBackedDrawingArea):
        (WebKit::LayerBackedDrawingArea::setSize):
        (WebKit::LayerBackedDrawingArea::showDebugBorders):
        (WebKit::LayerBackedDrawingArea::showRepaintCounter):
        * WebProcess/WebPage/LayerBackedDrawingArea.h:
        * WebProce...