This method is only used in a slow dcheck, thus omit it completely
otherwise.

R=ahaas@chromium.org

Bug: v8:8238
Change-Id: Ic23d0ff10a1dfe9f383237c99a365c2d3ee93e51
Reviewed-on: https://chromium-review.googlesource.com/c/1349233Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57781}