This flag will be used to toggle things for isolate-independent
builtins during development.

Bug: v8:6666
Change-Id: I8a97f08b3d677a01a2a55a4c6445e71e74471f51
Reviewed-on: https://chromium-review.googlesource.com/924067
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51325}