This reverts commit c7eed83f.

Reason for revert: I'm getting reports of internal iOS checkouts being broken. Reverting while I reproduce / debug it.

Original change's description:
> Add a Str() function to gclient for use in DEPS files.
> 
> 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.
> 
> Bug: 1099242
> 
> Change-Id: Ic2a17eea5f7098113bdba0557fe29e1a931a74b8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2268406
> Reviewed-by: Ben Pastene <bpastene@chromium.org>
> Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@google.com>

TBR=thakis@chromium.org,dpranke@google.com,ehmaldonado@chromium.org,bpastene@chromium.org,apolito@google.com,infra-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: Iac2b003f32acdbca15a19f821b61423e34b3466c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1099242
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2273978Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>