Replacing hard-coded clang version by a common variable.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index dac6163f280f2f112c137f94830255347b2cfbf4..2dddba6bc15a479bb923892880fb5130bdd1781e 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -17,6 +17,14 @@ declare_args() {
   # LLVM toolchain (see llvm_force_head_revision in
   # build/config/compiler/BUILD.gn).
   use_lld = false
+
+  # Clang compiler version. Clang files are placed at version-dependent paths.

[brettw, 2016/05/03 18:03:25]

I was thinking:
  declare_args() {
    ...

    if (is_clang) {
      # Clang compiler version. Clang files are placed at version-dependent
paths.
      clang_version = "3.9.0"
    }
  }

So this won't even be declared as a build arg, or defined at all, unless you're
actually using clang.

[aizatsky, 2016/05/03 18:07:31]

Done. I didn't know conditionals are allowed in declare_args.

+  # It is set to a specific value below only when is_clang==true.
+  clang_version = "undefined"
+}
+
+if (is_clang) {
+  clang_version = "3.9.0"
 }
 
 # Subdirectory within root_out_dir for shared library files.