This change enables moving the WasmToJSWrapper off the GCed heap

R=bradnelson@chromium.org,mtrofin@chromium.org,clemensh@chromium.org

Bug: 
Change-Id: I1ea8810fa7b71bcb2a80a1390742db64872c5bb1
Reviewed-on: https://chromium-review.googlesource.com/665977Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Aseem Garg <aseemgarg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48118}