Sadly the Nexus 6P doesn't support any kind of direct video-out (USB-C, HDMI or otherwise). It came out before the USB-C screen mirroring specs were finalized. All you can do is chromecast over the network.
But hopefully by this time next year, new cell phones will generally support screen mirroring over USB-C. Some phones (like the LG G5 [1]) already support it.
