This just fixes the text for the '--liftoff' and '--trace-liftoff'
flags to avoid calling Liftoff "experimental".

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: Iff9032070fe0e18ee77debf376f5453d1a61480e
Reviewed-on: https://chromium-review.googlesource.com/1126114Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54233}