Make it possible to refer to built-in variables without having to declare
then in DEPS files.

Bug: None
Change-Id: I5403963052463befc074f29750de56cce13927ce
Reviewed-on: https://chromium-review.googlesource.com/c/1312234Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>