Before the existence of "= delete", we were enforcing that the
DISALLOW_* macros were used in the private: section of classes only.
This is not needed any more, hence remove the comment on the macros.

Also, introduce macros for making types move-only, and use them
instead of our special macro in wasm.

R=bmeurer@chromium.org
CC=titzer@chromium.org

Change-Id: Iceba456fb0a32ae67defe16e35b865db8c8da500
Reviewed-on: https://chromium-review.googlesource.com/577687Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46773}