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

tools/gn/parser.h

Index: tools/gn/parser.h
diff --git a/tools/gn/parser.h b/tools/gn/parser.h
index 530596f657b53eacf1393fbdd7794c3b884f6c1b..3b5c90005066e6e4917db16f1deaaff5ae4367ca 100644
--- a/tools/gn/parser.h
+++ b/tools/gn/parser.h
@@ -19,6 +19,8 @@ typedef scoped_ptr<ParseNode> (Parser::*PrefixFunc)(Token token);
 typedef scoped_ptr<ParseNode> (Parser::*InfixFunc)(scoped_ptr<ParseNode> left,
                                                    Token token);
 
+extern const char kSyntax_Help[];
+
 struct ParserHelper {
   PrefixFunc prefix;
   InfixFunc infix;
@@ -64,11 +66,9 @@ class Parser {
   scoped_ptr<ParseNode> Subscript(scoped_ptr<ParseNode> left, Token token);
   scoped_ptr<ParseNode> DotOperator(scoped_ptr<ParseNode> left, Token token);
 
-  // Helper to parse a comma separated list, optionally allowing trailing
-  // commas (allowed in [] lists, not in function calls).
+  // Helper to parse a comma separated list.
   scoped_ptr<ListNode> ParseList(Token start_token,
-                                 Token::Type stop_before,
-                                 bool allow_trailing_comma);
+                                 Token::Type stop_before);
 
   scoped_ptr<ParseNode> ParseFile();
   scoped_ptr<ParseNode> ParseStatement();