This is a reland of 88f9c40e

Original change's description:
> gclient eval: Expand vars while parsing DEPS files
> 
> Introduce a Parse function that takes care of expanding vars while parsing
> the DEPS file.
> 
> It wraps Exec and exec calls, and supports deferring the expansion until
> later, so gclient flatten gets access to the unexpanded version.
> 
> Bug: 821199
> Change-Id: I943b021cc4474c9cda67b3816b841dd8ada3f5b2
> Reviewed-on: https://chromium-review.googlesource.com/973749
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Aaron Gable <agable@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>

Bug: 821199
Change-Id: Ic04af0cae59abc01a0382e2de3497a91cf7e62fd
Reviewed-on: https://chromium-review.googlesource.com/978561Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>