Fix GN Windows size-truncation warning.

tools/gn/setup.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/setup.h"
 
 #include <stdlib.h>
 
 #include <algorithm>
 #include <sstream>
@@ skipping 358 matching lines @@
   base::FilePath build_arg_file =
       build_settings_.GetFullPath(GetBuildArgFile());
   base::CreateDirectory(build_arg_file.DirName());
 
   std::string contents = stream.str();
 #if defined(OS_WIN)
   // Use Windows lineendings for this file since it will often open in
   // Notepad which can't handle Unix ones.
   ReplaceSubstringsAfterOffset(&contents, 0, "\n", "\r\n");
 #endif
-  if (base::WriteFile(build_arg_file, contents.c_str(), contents.size()) ==
-      -1) {
+  if (base::WriteFile(build_arg_file, contents.c_str(),
+      static_cast<int>(contents.size())) == -1) {
     Err(Location(), "Args file could not be written.",
       "The file is \"" + FilePathToUTF8(build_arg_file) +
         "\"").PrintToStdout();
     return false;
   }
 
   return true;
 }
 
 bool Setup::FillSourceDir(const CommandLine& cmdline) {
@@ skipping 200 matching lines @@
 }
 
 void DependentSetup::RunPreMessageLoop() {
   CommonSetup::RunPreMessageLoop();
 }
 
 bool DependentSetup::RunPostMessageLoop() {
   return CommonSetup::RunPostMessageLoop();
 }