This is a reland of 183bc643.

Original change's description:
> [wasm] Fix page size constant + test limits on max pages.
> 
> BUG=chromium:801604
> R=bbudge@chromium.org,titzer@chromium.org
> 
> Change-Id: I5fa57d5f8ea288d7aef7d466cbf11df46c4dcd1d
> Reviewed-on: https://chromium-review.googlesource.com/869468
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Reviewed-by: Bill Budge <bbudge@chromium.org>
> Commit-Queue: Brad Nelson <bradnelson@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50665}

Bug: chromium:801604
Change-Id: I89943644f5921f8635e62af82bb472c7bfd5e145
Reviewed-on: https://chromium-review.googlesource.com/874696Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50707}