Mark WindowImpl as noncopyable

Change-Id: I80f981216067a03578cfc812b006e6b68b77068e
Once ~SystemWindow calls ~Window, this is no longer a SystemWindow

...and hopefully it is just as fine to start this safety check at the paren,
anyway.

Change-Id: Ie3d7a1e233b9e3076a6603f2c6312d57b42b0f77