gn: Tweak precompiled_header docs.

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 221 matching lines @@
 extern const char kOutputPrefixOverride_Help[];
 
 extern const char kOutputs[];
 extern const char kOutputs_HelpShort[];
 extern const char kOutputs_Help[];
 
 extern const char kPrecompiledHeader[];
 extern const char kPrecompiledHeader_HelpShort[];
 extern const char kPrecompiledHeader_Help[];
 
+extern const char kPrecompiledHeaderType[];
+extern const char kPrecompiledHeaderType_HelpShort[];
+extern const char kPrecompiledHeaderType_Help[];
+
 extern const char kPrecompiledSource[];
 extern const char kPrecompiledSource_HelpShort[];
 extern const char kPrecompiledSource_Help[];
 
 extern const char kProductType[];
 extern const char kProductType_HelpShort[];
 extern const char kProductType_Help[];
 
 extern const char kPublic[];
 extern const char kPublic_HelpShort[];
@@ skipping 48 matching lines @@
 // 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_