tools/gn: Fix errors found by Facebook's flint tool.

tools/gn/standard_out.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/standard_out.h"
 
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ skipping 77 matching lines @@
         break;
       case DECORATION_YELLOW:
         ::SetConsoleTextAttribute(hstdout,
                                   FOREGROUND_RED | FOREGROUND_GREEN);
         break;
     }
   }
 
   DWORD written = 0;
   ::WriteFile(hstdout, output.c_str(), static_cast<DWORD>(output.size()),
-              &written, NULL);
+              &written, nullptr);
 
   if (is_console)
     ::SetConsoleTextAttribute(hstdout, default_attributes);
 }
 
 #else
 
 void OutputString(const std::string& output, TextDecoration dec) {
   EnsureInitialized();
   if (is_console) {
@@ skipping 77 matching lines @@
         break;
       } else if (line[char_i] != ' ') {
         break;
       }
     }
 
     OutputString(line + "\n", dec);
   }
 }