Add 'gn help grammar' contents to 'gn help all'.

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"
@@ skipping 102 matching lines @@
     PrintLongHelp(f.second.help);
 
   for (const auto& v: variables::GetBuiltinVariables())
     PrintLongHelp(v.second.help);
 
   for (const auto& v: variables::GetTargetVariables())
     PrintLongHelp(v.second.help);
 
   PrintLongHelp(kBuildArgs_Help);
   PrintLongHelp(kDotfile_Help);
+  PrintLongHelp(kGrammar_Help);
   PrintLongHelp(kInputConversion_Help);
   PrintLongHelp(kLabelPattern_Help);
   PrintLongHelp(kSourceExpansion_Help);
   PrintSwitchHelp();
 }
 
 // Prints help on the given switch. There should be no leading hyphens. Returns
 // true if the switch was found and help was printed. False means the switch is
 // unknown.
 bool PrintHelpOnSwitch(const std::string& what) {
@@ skipping 111 matching lines @@
     return 0;
   }
 
   // No help on this.
   Err(Location(), "No help on \"" + what + "\".").PrintToStdout();
   RunHelp(std::vector<std::string>());
   return 1;
 }
 
 }  // namespace commands