This both simplifies the code, and brings it in-line with
what we need for wasm on the native heap.

Bug: v8:6876
Change-Id: Ia48aed86ddcd94e85c0aecc053c327dfacb795d6
Reviewed-on: https://chromium-review.googlesource.com/744593
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49048}