Enforce both engine limitations and spec (http://asmjs.org/spec/latest/)
limitations on the size of asm.js heaps.

R=clemensh@chromium.org
CC=​mstarzinger@chromium.org

Bug: chromium:873600
Change-Id: I104c23bbd0a9a7c494f97f8f9e83ac5a37496dfd
Reviewed-on: https://chromium-review.googlesource.com/1174411
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55163}