This will act as a switch so that we can enable Node.js GN builds
via V8 change.

R=machenbach@chromium.org

Bug: v8:6105
Change-Id: Ie786a4dad7f53e4bbf7d8830bb72b1f8031cee12
Reviewed-on: https://chromium-review.googlesource.com/866839Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50612}