Add GN function get_label_info

tools/gn/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_SETTINGS_H_
 #define TOOLS_GN_SETTINGS_H_
 
 #include "base/files/file_path.h"
 #include "tools/gn/build_settings.h"
 #include "tools/gn/import_manager.h"
@@ skipping 15 matching lines @@
 // the file with the toolchain declaration in it.
 class Settings {
  public:
   enum TargetOS {
     UNKNOWN,
     LINUX,
     MAC,
     WIN
   };
 
-  // Constructs a toolchain settings. The output_subdir_name is the name we
-  // should use for the subdirectory in the build output directory for this
-  // toolchain's outputs. It should have no slashes in it. The default
-  // toolchain should use an empty string.
+  // Constructs a toolchain settings.
+  //
+  // The output_subdir_name is the name we should use for the subdirectory in
+  // the build output directory for this toolchain's outputs. The default
+  // toolchain would use an empty string (it goes in the root build dir).
+  // Otherwise, it must end in a slash.
   Settings(const BuildSettings* build_settings,
            const std::string& output_subdir_name);
   ~Settings();
 
   const BuildSettings* build_settings() const { return build_settings_; }
 
   const Label& toolchain_label() const { return toolchain_label_; }
   void set_toolchain_label(const Label& l) { toolchain_label_ = l; }
 
   const Label& default_toolchain_label() const {
@@ skipping 70 matching lines @@
   SourceDir toolchain_gen_dir_;
 
   Scope base_config_;
 
   bool greedy_target_generation_;
 
   DISALLOW_COPY_AND_ASSIGN(Settings);
 };
 
 #endif  // TOOLS_GN_SETTINGS_H_