Allow overriding script executable in .gn file

tools/gn/setup.h

 // 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.
 
 #ifndef TOOLS_GN_SETUP_H_
 #define TOOLS_GN_SETUP_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 101 matching lines @@
   // Fills the root directory into the settings. Returns true on success.
   bool FillSourceDir(const base::CommandLine& cmdline);
 
   // Fills the build directory given the value the user has specified.
   // Must happen after FillSourceDir so we can resolve source-relative
   // paths. If require_exists is false, it will fail if the dir doesn't exist.
   bool FillBuildDir(const std::string& build_dir, bool require_exists);
 
   // Fills the python path portion of the command line. On failure, sets
   // it to just "python".
-  void FillPythonPath(const base::CommandLine& cmdline);
+  bool FillPythonPath(const base::CommandLine& cmdline);
 
   // Run config file.
   bool RunConfigFile();
 
   bool FillOtherConfig(const base::CommandLine& cmdline);
 
   BuildSettings build_settings_;
   scoped_refptr<LoaderImpl> loader_;
   Builder builder_;
 
@@ skipping 29 matching lines @@
   // this around for the entire run so that Values can blame to the command
   // line when we issue errors about them.
   std::unique_ptr<InputFile> args_input_file_;
   std::vector<Token> args_tokens_;
   std::unique_ptr<ParseNode> args_root_;
 
   DISALLOW_COPY_AND_ASSIGN(Setup);
 };
 
 #endif  // TOOLS_GN_SETUP_H_