gn: Support build directories outside the source tree.

tools/gn/output_file.cc

 // 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 "tools/gn/output_file.h"
 
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/source_file.h"
 
 OutputFile::OutputFile() : value_() {
 }
 
 OutputFile::OutputFile(const base::StringPiece& str)
     : value_(str.data(), str.size()) {
 }
 
 OutputFile::OutputFile(const BuildSettings* build_settings,
                        const SourceFile& source_file)
     : value_(RebaseSourceAbsolutePath(source_file.value(),
-                                      build_settings->build_dir())) {
+                                      build_settings->build_dir(),
+                                      build_settings->root_path_utf8())) {
 }
 
 OutputFile::~OutputFile() {
 }
 
 SourceFile OutputFile::AsSourceFile(const BuildSettings* build_settings) const {
   DCHECK(!value_.empty());
   DCHECK(value_[value_.size() - 1] != '/');
   return SourceFile(build_settings->build_dir().value() + value_);
 }
 
 SourceDir OutputFile::AsSourceDir(const BuildSettings* build_settings) const {
   if (!value_.empty()) {
     // Empty means the root build dir. Otherwise, we expect it to end in a
     // slash.
     DCHECK(value_[value_.size() - 1] == '/');
   }
   return SourceDir(build_settings->build_dir().value() + value_);
 }