This reduces confusion with GC write barrier. The word "barrier" is
reserved for GC write barrier and "fence" for memory ordering fence.

BUG=v8:6474

Change-Id: Ic4352f04430eaca742b72db1580ee0a42a1ffefb
Reviewed-on: https://chromium-review.googlesource.com/528103Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45813}