[comment.js] fix corner case for comment.js plugin

In the following text, position cursor at the very beginning of the first line, press shift+down to select the first line and press `ctrl+/` to uncomment it.
```js
// console.log(1);
// console.log(2);
```

I expect to get the following:
```js
console.log(1);
// console.log(2);
```

Instead, I get both lines uncommented:
```js
console.log(1);
console.log(2);
```

Downstream bug: https://crbug.com/414302