Invoke async method on remote object got by remote.getGlobal causes memory leak.

The following is the test case.

In the test case, after the ```callback``` has been called, the ```arrow function``` keep been strongly referenced by ```the remote module```，and cannot be gc collected.

```
// main process

global.foo = {bar: {
testCallback(cb){
cb && cb('testCallback');
}
}};
```

```
// renderer process

let bar = remote.getGlobal('foo').bar;

// create a big array, to make monitor the memory leak easily
let willLeakArray = [];
for (let i = 0; i < 1024 * 1024 * 10; i++) {
willLeakArray.push('' + i);
}
bar.testCallback((value)=> {
console.log('testCallback', value, willLeakArray.length );
})
```