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

tools/gn/parser_unittest.cc

 // 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.
 
 #include <iostream>
 #include <sstream>
 
 #include "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/input_file.h"
 #include "tools/gn/parser.h"
@@ skipping 106 matching lines @@
 
 TEST(Parser, FunctionCall) {
   DoExpressionPrintTest("foo()",
       "FUNCTION(foo)\n"
       " LIST\n");
   DoExpressionPrintTest("blah(1, 2)",
       "FUNCTION(blah)\n"
       " LIST\n"
       "  LITERAL(1)\n"
       "  LITERAL(2)\n");
-  DoExpressionErrorTest("foo(1, 2,)", 1, 10);
   DoExpressionErrorTest("foo(1 2)", 1, 7);
 }
 
 TEST(Parser, ParenExpression) {
   const char* input = "(foo(1)) + (a + (b - c) + d)";
   const char* expected =
       "BINARY(+)\n"
       " FUNCTION(foo)\n"
       "  LIST\n"
       "   LITERAL(1)\n"
@@ skipping 556 matching lines @@
 // freestanding block.
 TEST(Parser, StandaloneBlock) {
   DoParserErrorTest(
       "if (true) {\n"
       "}\n"
       "{\n"
       "  assert(false)\n"
       "}\n",
       3, 1);
 }