Add gn Blink GC plugin options

build/toolchain/mac/BUILD.gn

 # Copyright (c) 2013 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.
 
 # TODO(brettw) Use "gcc_toolchain.gni" like the Linux toolchains. This requires
 # some enhancements since the commands on Mac are slightly different than on
 # Linux.
 
 import("../goma.gni")
+import("//build/config/clang/clang.gni")
 import("//build/config/ios/ios_sdk.gni")
 import("//build/config/mac/mac_sdk.gni")
 import("//build/config/mac/symbols.gni")
 
 assert(host_os == "mac")
 
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 import("//build/toolchain/concurrent_links.gni")
 
 if (use_goma) {
   assert(cc_wrapper == "", "Goma and cc_wrapper can't be used together.")
   compiler_prefix = "$goma_dir/gomacc "
 } else if (cc_wrapper != "") {
   compiler_prefix = cc_wrapper + " "
 } else {
   compiler_prefix = ""
 }
 
 if (!use_xcode_clang) {
-  compiler_prefix = compiler_prefix +
-                    rebase_path("//third_party/llvm-build/Release+Asserts/bin/",
-                                root_build_dir)
+  compiler_prefix =
+      compiler_prefix + rebase_path("$clang_base_path/bin/", root_build_dir)
 }
 
 if (current_toolchain == default_toolchain) {
   # Reduce the number of tasks using the copy_bundle_data and compile_xcassets
   # tools as they can cause lots of I/O contention when invoking ninja with a
   # large number of parallel jobs (e.g. when using distributed build like goma).
   pool("bundle_pool") {
     depth = concurrent_links
   }
 }
@@ skipping 326 matching lines @@
 
 mac_toolchain("ios_clang_x86") {
   toolchain_cpu = "x86"
   toolchain_os = "ios"
 }
 
 mac_toolchain("ios_clang_x64") {
   toolchain_cpu = "x64"
   toolchain_os = "ios"
 }