This prepares a reland of https://crrev.com/c/869468.

Drive-by: Add a static_assert, also to document why
kV8MaxWasmMemoryPages was chosed to be slightly below 2GB.

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

Bug: v8:6600
Change-Id: I6417bec191803c791fa5b218024ebcfde27e2aea
Reviewed-on: https://chromium-review.googlesource.com/873912Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50692}