macOS: Don't disable custom libc++ for cef_sandbox build (see issue #2677)

tools/gn_args.py

@@ -441,15 +441,16 @@ def GetConfigArgsSandbox(platform, args, is_debug, cpu):
 
       # Avoid /LTCG linker warnings and generate smaller lib files.
       'is_official_build': False,
-
-      # Avoid Debug build linker errors caused by custom libc++.
-      'use_custom_libcxx': False,
   }
 
   if is_debug:
     # Enable iterator debugging (_ITERATOR_DEBUG_LEVEL=2).
     add_args['enable_iterator_debugging'] = True
 
+  if platform == 'windows':
+    # Avoid Debug build linker errors caused by custom libc++.
+    add_args['use_custom_libcxx'] = False
+
   result = MergeDicts(args, add_args, {
       'is_debug': is_debug,
       'target_cpu': cpu,