Improve Windows GN build setup.

tools/gn/build_settings.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_BUILD_SETTINGS_H_
 #define TOOLS_GN_BUILD_SETTINGS_H_
 
 #include <map>
 
 #include "base/basictypes.h"
@@ skipping 42 matching lines @@
 
   // Path of the python executable to run scripts with.
   base::FilePath python_path() const { return python_path_; }
   void set_python_path(const base::FilePath& p) { python_path_ = p; }
 
   const SourceFile& build_config_file() const { return build_config_file_; }
   void set_build_config_file(const SourceFile& f) { build_config_file_ = f; }
 
   // The build directory is the root of all output files. The default toolchain
   // files go into here, and non-default toolchains will have separate
-  // toolchain-specific root directories inside this.
+  // toolchain-specific root directories inside this. This should be set after
+  // the root path for the proper directory to be created.
   const SourceDir& build_dir() const { return build_dir_; }
   void SetBuildDir(const SourceDir& dir);
 
   // The inverse of relative_build_dir, ending with a separator.
   // Example: relative_build_dir_ = "out/Debug/" this will be "../../"
   const std::string& build_to_source_dir_string() const {
     return build_to_source_dir_string_;
   }
 
   // The build args are normally specified on the command-line.
@@ skipping 48 matching lines @@
 
   // See getters above.
   mutable ItemTree item_tree_;
   mutable TargetManager target_manager_;
   mutable ToolchainManager toolchain_manager_;
 
   BuildSettings& operator=(const BuildSettings& other);  // Disallow.
 };
 
 #endif  // TOOLS_GN_BUILD_SETTINGS_H_