tools/gn: add "gn help grammar" with formal grammar

tools/gn/parser.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_PARSER_H_
 #define TOOLS_GN_PARSER_H_
 
 #include <map>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/scoped_ptr.h"
 #include "tools/gn/err.h"
 #include "tools/gn/parse_tree.h"
 
 class Parser;
 typedef scoped_ptr<ParseNode> (Parser::*PrefixFunc)(Token token);
 typedef scoped_ptr<ParseNode> (Parser::*InfixFunc)(scoped_ptr<ParseNode> left,
                                                    Token token);
 
+extern const char kGrammar_Help[];
+
 struct ParserHelper {
   PrefixFunc prefix;
   InfixFunc infix;
   int precedence;
 };
 
 // Parses a series of tokens. The resulting AST will refer to the tokens passed
 // to the input, so the tokens an the file data they refer to must outlive your
 // use of the ParseNode.
 class Parser {
@@ skipping 85 matching lines @@
   FRIEND_TEST_ALL_PREFIXES(Parser, Expression);
   FRIEND_TEST_ALL_PREFIXES(Parser, FunctionCall);
   FRIEND_TEST_ALL_PREFIXES(Parser, List);
   FRIEND_TEST_ALL_PREFIXES(Parser, ParenExpression);
   FRIEND_TEST_ALL_PREFIXES(Parser, UnaryOp);
 
   DISALLOW_COPY_AND_ASSIGN(Parser);
 };
 
 #endif  // TOOLS_GN_PARSER_H_