Add testonly flag to GN

tools/gn/target_generator.cc

Index: tools/gn/target_generator.cc
diff --git a/tools/gn/target_generator.cc b/tools/gn/target_generator.cc
index 657581ac0166560067eaee51ebae4f3dc68920b2..02c055e336e748a97af486b22ba1697e77eeaeb8 100644
--- a/tools/gn/target_generator.cc
+++ b/tools/gn/target_generator.cc
@@ -48,6 +48,10 @@ void TargetGenerator::Run() {
   if (err_->has_error())
     return;
 
+  FillTestonly();
+  if (err_->has_error())
+    return;
+
   if (!Visibility::FillItemVisibility(target_, scope_, err_))
     return;
 
@@ -217,6 +221,15 @@ void TargetGenerator::FillDependencies() {
     return;
 }
 
+void TargetGenerator::FillTestonly() {
+  const Value* value = scope_->GetValue(variables::kTestonly, true);
+  if (value) {
+    if (!value->VerifyTypeIs(Value::BOOLEAN, err_))
+      return;
+    target_->set_testonly(value->boolean_value());
+  }
+}
+
 void TargetGenerator::FillOutputs(bool allow_substitutions) {
   const Value* value = scope_->GetValue(variables::kOutputs, true);
   if (!value)