GN: Add ability to specify a depfile for custom targets.

tools/gn/script_values.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_SCRIPT_VALUES_H_
 #define TOOLS_GN_SCRIPT_VALUES_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 14 matching lines @@
   // Arguments to the script.
   std::vector<std::string>& args() { return args_; }
   const std::vector<std::string>& args() const { return args_; }
   void swap_in_args(std::vector<std::string>* a) { args_.swap(*a); }
 
   // Files created by the script.
   std::vector<SourceFile>& outputs() { return outputs_; }
   const std::vector<SourceFile>& outputs() const { return outputs_; }
   void swap_in_outputs(std::vector<SourceFile>* op) { outputs_.swap(*op); }
 
+  // Depfile generated by the script.
+  const SourceFile& depfile() const { return depfile_; }
+  bool has_depfile() const { return !depfile_.is_null(); }
+  void set_depfile(const SourceFile& depfile) { depfile_ = depfile; }
+
  private:
   SourceFile script_;
   std::vector<std::string> args_;
   std::vector<SourceFile> outputs_;
+  SourceFile depfile_;
 
   DISALLOW_COPY_AND_ASSIGN(ScriptValues);
 };
 
 #endif  // TOOLS_GN_SCRIPT_VALUES_H_