tools/gn: mark first TestTarget parameter as const

tools/gn/test_with_scope.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/test_with_scope.h"
 
 #include "base/bind.h"
 #include "tools/gn/parser.h"
 #include "tools/gn/tokenizer.h"
 
@@ skipping 138 matching lines @@
   input_file_.SetContents(input);
 
   tokens_ = Tokenizer::Tokenize(&input_file_, &parse_err_);
   if (!parse_err_.has_error())
     parsed_ = Parser::Parse(tokens_, &parse_err_);
 }
 
 TestParseInput::~TestParseInput() {
 }
 
-TestTarget::TestTarget(TestWithScope& setup,
+TestTarget::TestTarget(const TestWithScope& setup,
                        const std::string& label_string,
                        Target::OutputType type)
     : Target(setup.settings(), setup.ParseLabel(label_string)) {
   visibility().SetPublic();
   set_output_type(type);
   SetToolchain(setup.toolchain());
 }
 
 TestTarget::~TestTarget() {
 }