This is an exact reland of https://chromium-review.googlesource.com/583617 .

One of the main use cases is making it clear which revision hashes
need to be changed together. The way it's usually done is one variable
referenced several times. With this CL, we preserve the references
from original DEPS, as opposed to evaluating them and losing some info.

This CL actually makes Var() emit a variable placeholder
instead of its value, and adds support for these placeholders
to gclient.

One of possible next steps might be to deprecate Var().

Bug: 570091, 748486
Change-Id: Id47e3771b7163149a4cd427b84f84ece52772f34
Reviewed-on: https://chromium-review.googlesource.com/586594Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: William Hesse <whesse@google.com>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>