This refactors the {AsmJs} methods used for instantiating an asm.js
module to only use one single entry point. It is in preparation to
validate the "memory" argument as well.

R=clemensh@chromium.org
BUG=chromium:715505

Change-Id: I5e26fcf46f98c053080c70b26c0f562afc7f794a
Reviewed-on: https://chromium-review.googlesource.com/488226
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44922}