GN: Use the correct defaults for templates invoked via target().

tools/gn/template.h

Index: tools/gn/template.h
diff --git a/tools/gn/template.h b/tools/gn/template.h
index a79d82f368ba3414a6b36c1f8b02a1f46612f02d..da5a1a458e8b8b84273740d933f59c617c7ab568 100644
--- a/tools/gn/template.h
+++ b/tools/gn/template.h
@@ -33,9 +33,12 @@ class Template : public base::RefCountedThreadSafe<Template> {
   Template(std::unique_ptr<Scope> closure, const FunctionCallNode* def);
 
   // Invoke the template. The values correspond to the state of the code
-  // invoking the template.
+  // invoking the template. The template name needs to be supplied since the
+  // template object itself doesn't know what name the calling code is using
+  // to refer to it (this is used to set defaults).
   Value Invoke(Scope* scope,
                const FunctionCallNode* invocation,
+               const std::string& template_name,
                const std::vector<Value>& args,
                BlockNode* block,
                Err* err) const;