GN: Change gen command syntax, support relative dirs.

tools/gn/ninja_build_writer.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 "tools/gn/ninja_build_writer.h"
 
 #include <fstream>
 
 #include "base/command_line.h"
 #include "base/file_util.h"
@@ skipping 13 matching lines @@
 #include <windows.h>
 #endif
 
 namespace {
 
 std::string GetSelfInvocationCommand(const BuildSettings* build_settings) {
   base::FilePath executable;
   PathService::Get(base::FILE_EXE, &executable);
 
   CommandLine cmdline(executable);
+  cmdline.AppendArg("gen");
+  cmdline.AppendArg(build_settings->build_dir().value());
   cmdline.AppendSwitchPath("--root", build_settings->root_path());
   cmdline.AppendSwitch("-q");  // Don't write output.
 
   EscapeOptions escape_shell;
   escape_shell.mode = ESCAPE_SHELL;
 #if defined(OS_WIN)
   // The command line code quoting varies by platform. We have one string,
   // possibly with spaces, that we want to quote. The Windows command line
   // quotes again, so we don't want quoting. The Posix one doesn't.
   escape_shell.inhibit_quoting = true;
@@ skipping 146 matching lines @@
       all_rules.append(" $\n    ");
     all_rules.append(target_file.value());
   }
 
   if (!all_rules.empty()) {
     out_ << "\nbuild all: phony " << all_rules << std::endl;
     out_ << "default all" << std::endl;
   }
 }