Move clang_version setting back into declare_args

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index f389eec29e41bf0d97c8a8ade6b867d497fe386b..fbaf48066840ae7f51436e87ff8e52b23091fc4f 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -36,14 +36,16 @@ declare_args() {
   use_xcode_clang = is_ios && is_official_build
 }
 
-if (is_clang) {
-  # Clang compiler version. Clang files are placed at version-dependent paths.
-  clang_version = "3.9.0"
+declare_args() {
+  if (is_clang) {
+    # Clang compiler version. Clang files are placed at version-dependent paths.
+    clang_version = "3.9.0"
 
-  if (llvm_force_head_revision) {
-    # TODO(hans): Remove after next Clang roll.
-    # ToT Clang has a higher version number.
-    clang_version = "4.0.0"
+    if (llvm_force_head_revision) {
+      # TODO(hans): Remove after next Clang roll.
+      # ToT Clang has a higher version number.
+      clang_version = "4.0.0"
+    }
   }
 }