Use C++11 nullptr in CEF helper process in the sample

This fixes its compilation under Mac and also avoids triggering the
check forbidding the use of NULL in the new code.

samples/webview/cef_process_helper.cpp

@@ -15,5 +15,5 @@ int main(int argc, char* argv[]) {
   CefMainArgs main_args(argc, argv);
 
   // Execute the secondary process.
-  return CefExecuteProcess(main_args, NULL, NULL);
+  return CefExecuteProcess(main_args, nullptr, nullptr);
 }