Downgrade Mac GN arg save_unstripped_output to just a variable.

build/config/mac/symbols.gni

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/chrome_build.gni")
 
 # This file declares arguments and configs that control whether dSYM debug
 # info is produced and whether build products are stripped.
 
 declare_args() {
   # Produce dSYM files for targets that are configured to do so. dSYM
   # generation is controlled globally as it is a linker output (produced via
   # the //build/toolchain/mac/linker_driver.py. Enabling this will result in
   # all shared library, loadable module, and executable targets having a dSYM
   # generated.
   enable_dsyms = is_official_build
 
   # Strip symbols from linked targets by default. If this is enabled, the
   # //build/config/mac:strip_all config will be applied to all linked targets.
   # If custom stripping paramters are required, remove that config from a
   # linked target and apply custom -Wcrl,strip flags. See
   # //build/toolchain/mac/linker_driver.py for more information.
   enable_stripping = is_official_build
-
-  # Save unstripped copies of targets with a ".unstripped" suffix. This is
-  # useful to preserve the original output when enable_stripping=true.
-  save_unstripped_output = false
 }
 
-if (save_unstripped_output) {
-  assert(enable_stripping,
-         "save_unstripped_output=true requires enable_stripping=true")
-}
+# Save unstripped copies of targets with a ".unstripped" suffix. This is
+# useful to preserve the original output when enable_stripping=true but
+# we're not actually generating real dSYMs.
+save_unstripped_output = enable_stripping && !enable_dsyms