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

tools/gn/command_help.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 <algorithm>
 #include <iostream>
 
 #include "base/command_line.h"
 #include "tools/gn/args.h"
 #include "tools/gn/commands.h"
 #include "tools/gn/err.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/input_conversion.h"
 #include "tools/gn/label_pattern.h"
+#include "tools/gn/parser.h"
 #include "tools/gn/setup.h"
 #include "tools/gn/standard_out.h"
 #include "tools/gn/substitution_writer.h"
 #include "tools/gn/switches.h"
 #include "tools/gn/variables.h"
 
 namespace commands {
 
 namespace {
 
@@ skipping 30 matching lines @@
   for (const auto& target : variables::GetTargetVariables())
     PrintShortHelp(target.second.help_short);
 
   OutputString("\nOther help topics:\n");
   PrintShortHelp("buildargs: How build arguments work.");
   PrintShortHelp("dotfile: Info about the toplevel .gn file.");
   PrintShortHelp("label_pattern: Matching more than one label.");
   PrintShortHelp(
       "input_conversion: Processing input from exec_script and read_file.");
   PrintShortHelp("source_expansion: Map sources to outputs for scripts.");
+  PrintShortHelp("syntax: Formal syntax for GN build files.");
   PrintShortHelp("switches: Show available command-line switches.");
 }
 
 void PrintSwitchHelp() {
   OutputString("Available global switches\n", DECORATION_YELLOW);
   OutputString(
       "  Do \"gn help --the_switch_you_want_help_on\" for more. Individual\n"
       "  commands may take command-specific switches not listed here. See the\n"
       "  help on your specific command for more.\n\n");
 
@@ skipping 99 matching lines @@
     return 0;
   }
   if (what == "label_pattern") {
     PrintLongHelp(kLabelPattern_Help);
     return 0;
   }
   if (what == "source_expansion") {
     PrintLongHelp(kSourceExpansion_Help);
     return 0;
   }
+  if (what == "syntax") {
+    PrintLongHelp(kSyntax_Help);
+    return 0;
+  }
   if (what == "switches") {
     PrintSwitchHelp();
     return 0;
   }
 
   // No help on this.
   Err(Location(), "No help on \"" + what + "\".").PrintToStdout();
   RunHelp(std::vector<std::string>());
   return 1;
 }
 
 }  // namespace commands