The flag is only available in d8 and should therefore not be in
flag-definitions.h.

R=clemensh@chromium.org

Bug: v8:8238
Change-Id: Idd69914cea03e736cf3b156b5961d583a7b5352c
Reviewed-on: https://chromium-review.googlesource.com/c/1349244
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57829}