This brings the wasm-constants.js file inline with that (forked copy) in
the WebAssembly spec repo, which should make it easier to export tests
from V8 to the spec in the future.

R=clemensh@chromium.org

Bug: 
Change-Id: I7db23efc2d671f65b614f9dbc97ae2f355f91b04
Reviewed-on: https://chromium-review.googlesource.com/586248Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46894}