This is a reland of 5705acab

If a gclient config is not found, then built-in variables wont
be supported.

Original change's description:
> [gclient] Make getdep and setdep to provide builtin vars
>
> Bug: 906114
> Change-Id: I069cc21343911f7fdb3c91ecbd8fcba53fc8099f
> Reviewed-on: https://chromium-review.googlesource.com/c/1340461
> Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
> Commit-Queue: Eric Boren <borenet@chromium.org>

Bug: 906114
Change-Id: I72f30d10b5f0180fd5c616a42393f5b12055ce8e
Reviewed-on: https://chromium-review.googlesource.com/c/1341039Reviewed-by: Eric Boren <borenet@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>