This also makes the {AddCodeCopy} method more specific to only apply to
import wrappers, otherwise the use of {set_code} would be unprotected.

R=clemensh@chromium.org
BUG=v8:8015

Change-Id: I62561560f57e4cc235a338c0e769e50ff55ec42d
Reviewed-on: https://chromium-review.googlesource.com/1238477Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56137}