gn: define and use clang_base_path

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/clang/clang.gni")
 import("//build/config/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 import("//build/toolchain/concurrent_links.gni")
 
 # This template defines a toolchain for something that works like gcc
 # (including clang).
 #
@@ skipping 488 matching lines @@
          "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("$clang_base_path/bin", root_build_dir)
     cc = "$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",
                              "strip",
                            ])
 
     if (defined(invoker.use_debug_fission)) {
       use_debug_fission = invoker.use_debug_fission
     }
   }
 }