This relands c7eed83f with a fix to the way variables are
propagated from parent dependencies into child dependencies.

The original CL description from c7eed83f was:
> gclient's existing functionality for handling variables is
> ambiguous: the value of a variable can either be a string literal
> or an expression fragment. The implementation is required to
> parse a value as an expression, and, if it is legal, treat it
> as an expression instead of a literal. This means that
>
>   gclient_gn_args_file = 'src/build/args.gni'
>   gclient_gn_args = ['xcode_version']
>   vars = {
>     'xcode_version': 'xcode-12'
>   }
>
> would cause a problem because gclient would try to parse the
> variable as an expression, and 'xcode' would not be defined.
>
> This patch adds a workaround for this, where you can instead
> use the Str() function to explicitly tell gclient to treat the
> value as a string and not a potential expression.
>
> The above example would be changed to:
>
>   gclient_gn_args_file = 'src/build/args.gni'
>   gclient_gn_args = ['xcode_version']
>   vars = {
>     'xcode_version': Str('xcode-12')
>   }
>
> The variable may still be used in every context where it was legal
> to be used before.
>
This reverts commit 84431987dd384c79c84515004d19db67345a1c00.

Bug: 1099242
TBR=ehmaldonado@chromium.org

Change-Id: I047b871df47c367c1f34a3985e5813504e3c5c6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2274152
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>