Add an output_dir override to GN.

tools/gn/tool.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.
 
 #ifndef TOOLS_GN_TOOL_H_
 #define TOOLS_GN_TOOL_H_
 
 #include <string>
 
 #include "base/logging.h"
@@ skipping 34 matching lines @@
   // Should include a leading "." if nonempty.
   const std::string& default_output_extension() const {
     return default_output_extension_;
   }
   void set_default_output_extension(const std::string& ext) {
     DCHECK(!complete_);
     DCHECK(ext.empty() || ext[0] == '.');
     default_output_extension_ = ext;
   }
 
+  const SubstitutionPattern& default_output_dir() const {
+    return default_output_dir_;
+  }
+  void set_default_output_dir(const SubstitutionPattern& dir) {
+    DCHECK(!complete_);
+    default_output_dir_ = dir;
+  }
+
   // Dependency file (if supported).
   const SubstitutionPattern& depfile() const {
     return depfile_;
   }
   void set_depfile(const SubstitutionPattern& df) {
     DCHECK(!complete_);
     depfile_ = df;
   }
 
   DepsFormat depsformat() const {
@@ skipping 114 matching lines @@
 
   // Substitutions required by this tool.
   const SubstitutionBits& substitution_bits() const {
     DCHECK(complete_);
     return substitution_bits_;
   }
 
  private:
   SubstitutionPattern command_;
   std::string default_output_extension_;
+  SubstitutionPattern default_output_dir_;
   SubstitutionPattern depfile_;
   DepsFormat depsformat_;
   PrecompiledHeaderType precompiled_header_type_;
   SubstitutionPattern description_;
   std::string lib_switch_;
   std::string lib_dir_switch_;
   SubstitutionList outputs_;
   SubstitutionPattern link_output_;
   SubstitutionPattern depend_output_;
   SubstitutionPattern runtime_link_output_;
   std::string output_prefix_;
   bool restat_;
   SubstitutionPattern rspfile_;
   SubstitutionPattern rspfile_content_;
 
   bool complete_;
 
   SubstitutionBits substitution_bits_;
 
   DISALLOW_COPY_AND_ASSIGN(Tool);
 };
 
 #endif  // TOOLS_GN_TOOL_H_