Add option make_clang_dir to GN

build/toolchain/gcc_toolchain.gni

 # 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.
 
 import("//build/config/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/ccache.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 407 matching lines @@
         is_tsan = false
         is_ubsan = false
         is_ubsan_vptr = false
       }
     }
 
     forward_variables_from(invoker, [ "deps" ])
   }
 }
 
+declare_args() {
+  clang_dir = "//third_party/llvm-build/Release+Asserts"
+}
+
 # This is a shorthand for gcc_toolchain instances based on the
 # Chromium-built version of Clang.  Only the toolchain_cpu and
 # toolchain_os variables need to be specified by the invoker, and
 # optionally toolprefix if it's a cross-compile case.  Note that for
 # a cross-compile case this toolchain requires a config to pass the
 # appropriate -target option, or else it will actually just be doing
 # a native compile.  The invoker can optionally override use_gold too.
 template("clang_toolchain") {
   assert(defined(invoker.toolchain_cpu),
          "clang_toolchain() must specify a \"toolchain_cpu\"")
   assert(defined(invoker.toolchain_os),
          "clang_toolchain() must specify a \"toolchain_os\"")
   if (defined(invoker.toolprefix)) {
     toolprefix = invoker.toolprefix
   } else {
     toolprefix = ""
   }
 
   gcc_toolchain(target_name) {
-    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
-                         root_build_dir)
+    prefix = rebase_path(clang_dir + "/bin", root_build_dir)
     cc = "$prefix/clang"
     cxx = "$prefix/clang++"
     ld = cxx
     is_clang = true
 
     readelf = "${toolprefix}readelf"
     ar = "${toolprefix}ar"
     nm = "${toolprefix}nm"
 
     forward_variables_from(invoker,
                            [
                              "toolchain_cpu",
                              "toolchain_os",
                              "use_gold",
                            ])
   }
 }