Implement target visibility in GN

tools/gn/variables.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_VARIABLES_H_
 #define TOOLS_GN_VARIABLES_H_
 
 #include <map>
 
 #include "base/strings/string_piece.h"
@@ skipping 153 matching lines @@
 extern const char kScript_Help[];
 
 extern const char kSourcePrereqs[];
 extern const char kSourcePrereqs_HelpShort[];
 extern const char kSourcePrereqs_Help[];
 
 extern const char kSources[];
 extern const char kSources_HelpShort[];
 extern const char kSources_Help[];
 
+extern const char kVisibility[];
+extern const char kVisibility_HelpShort[];
+extern const char kVisibility_Help[];
+
 // -----------------------------------------------------------------------------
 
 struct VariableInfo {
   VariableInfo();
   VariableInfo(const char* in_help_short,
                const char* in_help);
 
   const char* help_short;
   const char* help;
 };
 
 typedef std::map<base::StringPiece, VariableInfo> VariableInfoMap;
 
 // Returns the built-in readonly variables.
 // Note: this is used only for help so this getter is not threadsafe.
 const VariableInfoMap& GetBuiltinVariables();
 
 // Returns the variables used by target generators.
 // Note: this is used only for help so this getter is not threadsafe.
 const VariableInfoMap& GetTargetVariables();
 
 }  // namespace variables
 
 #endif  // TOOLS_GN_VARIABLES_H_