This removes any kind of heap access from the asm.js validator internals
and hence makes it independent of a specific Isolate. It is a precursor
towards potentially being able to run validation while streaming.

R=clemensh@chromium.org

Change-Id: Ia3770bf03bb973b56de897b27be60d7e050af2c4
Reviewed-on: https://chromium-review.googlesource.com/518188
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45628}