Document GN invoker and target name variables.

tools/gn/template.cc

Index: tools/gn/template.cc
diff --git a/tools/gn/template.cc b/tools/gn/template.cc
index 3c4d76dafaef69bfa1d943fb163656ee448fc2c9..e865264174a1740c7fe8a9821dd8d18f5764b84c 100644
--- a/tools/gn/template.cc
+++ b/tools/gn/template.cc
@@ -12,6 +12,7 @@
 #include "tools/gn/scope.h"
 #include "tools/gn/scope_per_file_provider.h"
 #include "tools/gn/value.h"
+#include "tools/gn/variables.h"
 
 Template::Template(const Scope* scope, const FunctionCallNode* def)
     : closure_(scope->MakeClosure()),
@@ -77,14 +78,14 @@ Value Template::Invoke(Scope* scope,
   // Scope.SetValue will copy the value which will in turn copy the scope, but
   // if we instead create a value and then set the scope on it, the copy can
   // be avoided.
-  const char kInvoker[] = "invoker";
-  template_scope.SetValue(kInvoker, Value(nullptr, std::unique_ptr<Scope>()),
-                          invocation);
-  Value* invoker_value = template_scope.GetMutableValue(kInvoker, false);
+  template_scope.SetValue(variables::kInvoker,
+                          Value(nullptr, std::unique_ptr<Scope>()), invocation);
+  Value* invoker_value =
+      template_scope.GetMutableValue(variables::kInvoker, false);
   invoker_value->SetScopeValue(std::move(invocation_scope));
   template_scope.set_source_dir(scope->GetSourceDir());
 
-  const base::StringPiece target_name("target_name");
+  const base::StringPiece target_name(variables::kTargetName);
   template_scope.SetValue(target_name,
                           Value(invocation, args[0].string_value()),
                           invocation);
@@ -107,7 +108,7 @@ Value Template::Invoke(Scope* scope,
   // to overwrite the value of "invoker" and free the Scope owned by the
   // value. So we need to look it up again and don't do anything if it doesn't
   // exist.
-  invoker_value = template_scope.GetMutableValue(kInvoker, false);
+  invoker_value = template_scope.GetMutableValue(variables::kInvoker, false);
   if (invoker_value && invoker_value->type() == Value::SCOPE) {
     if (!invoker_value->scope_value()->CheckForUnusedVars(err))
       return Value();