Composited layers (the ones derived from LayerChromium) hold a raw pointer to the compositor (LayerRendererChromium) which means that if the compositor gets destroyed before the layers do they could be accessing uninitialized memory. One possible solution would be to make LayerChromium hold a reference to LayerRendererChromium instead of a raw pointer to ensure a proper destruction order.
Fixed in r68442: <http://trac.webkit.org/changeset/68442>