holding that, although Troxel does not so specifically require, California Constitution requires "grandparent seeking visitation rights over the objection of a fit parent to show by clear and convincing evidence that the parent's decision would be detrimental to the child"

concluding grandparent visitation statute not in per se violation of the Due Process Clause and requiring that the statute be construed to require grandparents to show by clear and convincing evidence that lack of visitation would be detrimental to the child

construing statutory presumption in favor of the parent's decision to require proof by clear and convincing evidence

limiting application of grandparent-visitation statute to cases showing harm by clear and convincing evidence