This makes sure that asm.js modules can only be instantiated with a
valid {ArrayBuffer} as the underlying heap buffer for all cases where
accepting anything else would be observably different from JavaScript
proper.

R=clemensh@chromium.org
TEST=mjsunit/asm/asm-memory
BUG=chromium:715505,chromium:715748

Change-Id: I355686200151c5667bf836824de922d657a8d943
Reviewed-on: https://chromium-review.googlesource.com/488521
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44938}