Now that we no longer clone {WasmCode} objects, the referenced protected
instructions became unique to each such object. We no longer need to
maintain a reference count on the protected instructions.

R=clemensh@chromium.org

Change-Id: Iaa5b9cd4b56cc06d75f7d0b71429b6147378c2ca
Reviewed-on: https://chromium-review.googlesource.com/1034061
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52881}