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

tools/gn/command_help.cc

Index: tools/gn/command_help.cc
diff --git a/tools/gn/command_help.cc b/tools/gn/command_help.cc
index 0f8c34790a7d593dff32f67014c13b532fbdc3a9..8c070c9c0bde012e97be364751c44bdfd6ece9bf 100644
--- a/tools/gn/command_help.cc
+++ b/tools/gn/command_help.cc
@@ -12,6 +12,7 @@
 #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"
@@ -62,6 +63,7 @@ void PrintToplevelHelp() {
   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.");
 }
 
@@ -181,6 +183,10 @@ int RunHelp(const std::vector<std::string>& args) {
     PrintLongHelp(kSourceExpansion_Help);
     return 0;
   }
+  if (what == "syntax") {
+    PrintLongHelp(kSyntax_Help);
+    return 0;
+  }
   if (what == "switches") {
     PrintSwitchHelp();
     return 0;