Wire up the close box in mojo_shell

All the code appears to be written so that when the
native viewport is destroyed (close box clicked) mojo
should tear itself down, but DisplayManager() was ignoring
one of its parameters during construction.

I've fixed DisplayManager to not ignore its callback
parameter, but now we crash on destruction (crbug.com/430317)
which we can now fix separately.

BUG=430317
R=sky@chromium.org

Review URL: https://codereview.chromium.org/701963002