GN: Capture and print stderr from exec_script

tools/gn/exec_process.h

 // Copyright 2014 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 "base/critical_closure.h"
-
-#import <UIKit/UIKit.h>
+#include <string>
 
 namespace base {
-namespace internal {
-
-bool IsMultiTaskingSupported() {
-  return [[UIDevice currentDevice] isMultitaskingSupported];
+class CommandLine;
+class FilePath;
 }
 
+namespace internal {
+bool ExecProcess(const base::CommandLine& cmdline,
+                 const base::FilePath& startup_dir,
+                 std::string* std_out,
+                 std::string* std_err,
+                 int* exit_code);
 }  // namespace internal
-}  // namespace base