Add a variable use_system_xcode to GN.

build/config/mac/base_rules.gni

Index: build/config/mac/base_rules.gni
diff --git a/build/config/mac/base_rules.gni b/build/config/mac/base_rules.gni
index 4e34cb53c8021772a414dcd32123671fb422b18f..266878ef7d0f829ea29977c4f1197f916c05d1dc 100644
--- a/build/config/mac/base_rules.gni
+++ b/build/config/mac/base_rules.gni
@@ -155,12 +155,18 @@ template("compile_xibs") {
     outputs = [
       "$target_gen_dir/$target_name/{{source_name_part}}.nib",
     ]
-    args =
-        [
-          "--input",
-          "{{source}}",
-          "--output",
-          rebase_path("$target_gen_dir/$target_name/{{source_name_part}}.nib"),
-        ] + ibtool_flags
+    args = [
+      "--input",
+      "{{source}}",
+      "--output",
+      rebase_path("$target_gen_dir/$target_name/{{source_name_part}}.nib"),
+    ]
+    if (!use_system_xcode) {
+      args += [
+        "--developer_dir",
+        hermetic_xcode_path,
+      ]
+    }
+    args += ibtool_flags
   }
 }