Fix GN Windows size-truncation warning.

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index cb1effaaab1ea3d883c688daaed9f3881af24a76..a3f0dcb2f9aca7f09320cbc17e82396c8a8c1d4e 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -376,8 +376,8 @@ bool Setup::SaveArgsToFile() {
   // 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();