Improve Windows GN build setup.

tools/gn/build_settings.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/build_settings.h"
 
+#include "base/file_util.h"
 #include "tools/gn/filesystem_utils.h"
 
 BuildSettings::BuildSettings()
     : using_external_generator_(false),
       item_tree_(),
       target_manager_(this),
       toolchain_manager_(this) {
 }
 
 BuildSettings::BuildSettings(const BuildSettings& other)
@@ skipping 21 matching lines @@
   root_path_utf8_ = FilePathToUTF8(root_path_);
 }
 
 void BuildSettings::SetSecondarySourcePath(const SourceDir& d) {
   secondary_source_path_ = GetFullPath(d);
 }
 
 void BuildSettings::SetBuildDir(const SourceDir& d) {
   build_dir_ = d;
   build_to_source_dir_string_ = InvertDir(d);
+
+  DCHECK(!root_path_.empty());
+  file_util::CreateDirectory(build_dir_.Resolve(root_path_));
 }
 
 base::FilePath BuildSettings::GetFullPath(const SourceFile& file) const {
   return file.Resolve(root_path_);
 }
 
 base::FilePath BuildSettings::GetFullPath(const SourceDir& dir) const {
   return dir.Resolve(root_path_);
 }
 
 base::FilePath BuildSettings::GetFullPathSecondary(
     const SourceFile& file) const {
   return file.Resolve(secondary_source_path_);
 }
 
 base::FilePath BuildSettings::GetFullPathSecondary(
     const SourceDir& dir) const {
   return dir.Resolve(secondary_source_path_);
 }