Add a --markdown flag to GN to be able to output help in markdown.

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 51 matching lines @@
   PrintShortHelp("dotfile: Info about the toplevel .gn file.");
   PrintShortHelp("grammar: Formal grammar for GN build files.");
   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("switches: Show available command-line switches.");
 }
 
 void PrintSwitchHelp() {
+  const base::CommandLine* cmdline = base::CommandLine::ForCurrentProcess();
+  bool use_markdown = cmdline->HasSwitch(switches::kMarkdown);
+
   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");
 
+  if (use_markdown)
+    OutputString("```\n\n", DECORATION_NONE);
+
   for (const auto& s : switches::GetSwitches())
     PrintShortHelp(s.second.short_help);
+
+  if (use_markdown)
+    OutputString("\n```\n", DECORATION_NONE);
 }
 
 void PrintAllHelp() {
-  PrintToplevelHelp();
+  const base::CommandLine* cmdline = base::CommandLine::ForCurrentProcess();
+  if (cmdline->HasSwitch(switches::kMarkdown)) {
+    OutputString("# GN Reference\n\n");
+    OutputString("[TOC]\n\n");
+    OutputString("*This page is automatically generated from* "
+                 "`gn help --markdown all`.\n\n");
+  } else {
+    PrintToplevelHelp();
+  }
 
   for (const auto& s : switches::GetSwitches()) {
     PrintLongHelp(s.second.long_help);
   }
 
   for (const auto& c: commands::GetCommands()) {
     PrintLongHelp(c.second.help);
   }
 
   for (const auto& f: functions::GetFunctions()) {
@@ skipping 136 matching lines @@
     return 0;
   }
 
   // No help on this.
   Err(Location(), "No help on \"" + what + "\".").PrintToStdout();
   RunHelp(std::vector<std::string>());
   return 1;
 }
 
 }  // namespace commands