Simplify getting callback

atom/common/api/atom_api_shell.cc

@@ -58,14 +58,11 @@ bool OpenExternal(
   }
 
   if (args->Length() >= 3) {
-    v8::Local<v8::Value> peek = args->PeekNext();
     platform_util::OpenExternalCallback callback;
-    if (mate::Converter<platform_util::OpenExternalCallback>::FromV8(
-      args->isolate(), peek, &callback)) {
+    if (args->GetNext(&callback)) {
       platform_util::OpenExternal(url, activate, callback);
       return true;
     }
-    return false;
   }
 
   return platform_util::OpenExternal(url, activate);