Reviewed by Sam Weinig.

        <rdar://problem/6988966> Hardware layers do not show up in page snapshots

        * WebView/WebHTMLViewPrivate.h:
        * WebView/WebHTMLView.mm:
        (-[WebHTMLView _compositingLayersHostingView]):
        Add a private method that returns the NSView used to host compositing layers.

        * platform/graphics/mac/Canvas3DLayer.h:
        * platform/graphics/mac/Canvas3DLayer.mm:
        (-[Canvas3DLayer copyImageSnapshotWithColorSpace:]):
        Add a method that gets called when snapshotting Canvas3DLayers for page snapshots,
        that allows the layer to return a CGImageRef of its contents.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@50067 268f45cc-cd09-0410-ab3c-d52691b4dbfc