gn: Support build directories outside the source tree.

tools/gn/ninja_build_writer.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/ninja_build_writer.h"
 
 #include <fstream>
 #include <map>
 
 #include "base/command_line.h"
@@ skipping 68 matching lines @@
     const Toolchain* default_toolchain,
     const std::vector<const Target*>& default_toolchain_targets,
     std::ostream& out,
     std::ostream& dep_out)
     : build_settings_(build_settings),
       all_settings_(all_settings),
       default_toolchain_(default_toolchain),
       default_toolchain_targets_(default_toolchain_targets),
       out_(out),
       dep_out_(dep_out),
-      path_output_(build_settings->build_dir(), ESCAPE_NINJA) {
+      path_output_(build_settings->build_dir(),
+                   build_settings->root_path_utf8(), ESCAPE_NINJA) {
 }
 
 NinjaBuildWriter::~NinjaBuildWriter() {
 }
 
 bool NinjaBuildWriter::Run(Err* err) {
   WriteNinjaRules();
   WriteLinkPool();
   WriteSubninjas();
   return WritePhonyAndAllRules(err);
@@ skipping 166 matching lines @@
   EscapeOptions ninja_escape;
   ninja_escape.mode = ESCAPE_NINJA;
 
   // Escape for special chars Ninja will handle.
   std::string escaped = EscapeString(phony_name, ninja_escape, NULL);
 
   out_ << "build " << escaped << ": phony ";
   path_output_.WriteFile(out_, target_file);
   out_ << std::endl;
 }