Add a variable use_system_xcode to GN.

third_party/WebKit/Source/core/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # 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")
 import("//third_party/WebKit/Source/bindings/bindings.gni")
 import("//third_party/WebKit/Source/bindings/modules/v8/generated.gni")
 import("//third_party/WebKit/Source/bindings/scripts/scripts.gni")
 import("//third_party/WebKit/Source/config.gni")
 import("//third_party/WebKit/Source/core/core.gni")
 import("//third_party/WebKit/Source/build/scripts/scripts.gni")
 import("//third_party/WebKit/Source/modules/modules_idl_files.gni")
@@ skipping 780 matching lines @@
   ]
   outputs = [
     "$blink_core_output_dir/{{source_name_part}}.cpp",
     "$blink_core_output_dir/{{source_name_part}}.h",
   ]
   args = [
     "{{source}}",
     rel_blink_core_gen_dir,
     bison_exe,
   ]
+  if (!use_system_xcode) {
+    args += [ hermetic_xcode_path ]
+  }
 
   deps = make_core_generated_deps
 }
 
 # Compiles the code generated by the targets above.
 target(core_link_small_target_type, "core_generated") {
   sources = bindings_core_v8_files
 
   # Targets from above that generate outputs that need to be compiled.
   # All sources declared as outputs from these targets will be compiled into this
@@ skipping 436 matching lines @@
   sources = [
     "html/parser/HTMLPreloadScannerFuzzer.cpp",
     "html/parser/TextResourceDecoderForFuzzing.h",
   ]
   seed_corpus = "//third_party/WebKit/LayoutTests/fast/parser"
   deps = [
     ":core",
     "../platform:blink_fuzzer_test_support",
   ]
 }