Add option make_clang_dir to GN

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index 38e5fb6d3e97d211417865fe4a6f6f91bfefb8af..3b139f6574cd4c03ef7d7ce226f96166be6395cf 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -425,6 +425,11 @@ template("gcc_toolchain") {
   }
 }
 
+declare_args() {
+  # changes the path to the clang executable
+  use_clang_prefix = "default"
+}
+
 # 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
@@ -446,6 +451,9 @@ template("clang_toolchain") {
   gcc_toolchain(target_name) {
     prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
                          root_build_dir)
+    if (use_clang_prefix != "default") {
+      prefix = use_clang_prefix

[dshwang, 2016/01/20 15:05:05]

how about naming similar to gyp 'make_clang_dir'?

[mkollaro, 2016/01/21 14:33:59]

Done.

+    }
     cc = "$prefix/clang"
     cxx = "$prefix/clang++"
     ld = cxx