This prepares for relanding:
https://codereview.chromium.org/2058033002/

All toplevel variables are visible in files including v8.gni.
This works around potential name clashes.

BUG=chromium:616034

Review-Url: https://codereview.chromium.org/2169443002
Cr-Commit-Position: refs/heads/master@{#37896}