Downgrade Mac GN arg save_unstripped_output to just a variable.

build/config/mac/symbols.gni

Index: build/config/mac/symbols.gni
diff --git a/build/config/mac/symbols.gni b/build/config/mac/symbols.gni
index b54003a2b3f445cefab7c519c1782baea1364125..8cec6955dcebda06218744ff7abd49eb3ac36db1 100644
--- a/build/config/mac/symbols.gni
+++ b/build/config/mac/symbols.gni
@@ -21,13 +21,9 @@ declare_args() {
   # 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