This is a reland of 88f9c40e

We no longer need the vars dict to be declared before vars can be used.
This was causing problems because gclient flatten printed vars after deps,
breaking the above assumption.

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: I583df23558f91871e1a2aa2574c20d35e54635f6
Reviewed-on: https://chromium-review.googlesource.com/981086
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>