Now that tables and stack frames properly root instances, there is no
longer any need to disallow mutations that could unroot instances
while their code is on the stack.

Bug: v8:7232
Change-Id: I907b9522ac12ad7a67fb4124774713b6b3b40bb7
Reviewed-on: https://chromium-review.googlesource.com/1007004
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52629}