[Refactor Xcode Objects] Allow extra attributes for native targets.

tools/gn/xcode_object.h

Index: tools/gn/xcode_object.h
diff --git a/tools/gn/xcode_object.h b/tools/gn/xcode_object.h
index f56de09a7806658b2cf4c870be1fddc332e6bdd5..31f989b6d02a19a1b33721b4b9077fe16fe16a29 100644
--- a/tools/gn/xcode_object.h
+++ b/tools/gn/xcode_object.h
@@ -288,7 +288,8 @@ class PBXProject : public PBXObject {
                        const std::string& type,
                        const std::string& output_name,
                        const std::string& output_type,
-                       const std::string& shell_script);
+                       const std::string& shell_script,
+                       const PBXAttributes& extra_attributes = PBXAttributes());
 
   void SetProjectDirPath(const std::string& project_dir_path);
   void SetProjectRoot(const std::string& project_root);