The previous design assumed we can't possibly have a cycle involving
an instance, however, we can. For example: a script can reference
an instance, which ends up referencing the native context because
of how we generate wasm-to-js wrappers; that references the global
object, which then references the script. A global handle to the
indirect function table can then root such a cycle. That means
the instance is never collected, which never deletes the global
handle.

This change addresses that by making the handles weak.

Bug: 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ief7263af83974bf96505a4fba65d162474fe7c7c
Reviewed-on: https://chromium-review.googlesource.com/653852
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Aseem Garg <aseemgarg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47909}