Add a variable use_system_xcode to GN.

third_party/WebKit/Source/core/BUILD.gn

Index: third_party/WebKit/Source/core/BUILD.gn
diff --git a/third_party/WebKit/Source/core/BUILD.gn b/third_party/WebKit/Source/core/BUILD.gn
index f76ebcd7fea6dd8f5172d0174f4cd4298642217e..9bc4bedd9e4bd3b72c0b8884c157206733a7e319 100644
--- a/third_party/WebKit/Source/core/BUILD.gn
+++ b/third_party/WebKit/Source/core/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/chromecast_build.gni")
+import("//build/toolchain/toolchain.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
@@ -803,6 +804,9 @@ action_foreach("make_core_generated_bison") {
     rel_blink_core_gen_dir,
     bison_exe,
   ]
+  if (!use_system_xcode) {
+    args += [ hermetic_xcode_path ]
+  }
 
   deps = make_core_generated_deps
 }