Add a `p:video` element under `p:sld/p:timing`.

        The element will refer to the specified *pic* element by its shape
        id, and cause the video play controls to appear for that video.

def _add_video_timing(self, pic):
        """Add a `p:video` element under `p:sld/p:timing`.

        The element will refer to the specified *pic* element by its shape
        id, and cause the video play controls to appear for that video.
        """
        sld = self._spTree.xpath('/p:sld')[0]
        childTnLst = sld.get_or_add_childTnLst()
        childTnLst.add_video(pic.shape_id)