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/+/2268406Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>