* Electron version: 1.4.15
* Operating system: Windows 10

### Expected behavior

The new [before-input-event](https://github.com/electron/electron/blob/master/docs/api/web-contents.md#event-before-input-event) event should imitate the properties from [KeyboardEvents](https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent).

### Actual behavior

Missing the ```KeyboardEvent.code``` equivalent.
Because of this, it's not possible for me to distinguish between my shortcuts created with 1 and Numpad1 for instance.

Looking at comments for the ```before-input-event``` issue ([#7586](https://github.com/electron/electron/issues/7586#issuecomment-263431783)), The ```code``` property was never mentioned in the psudo code. Missed by mistake?

```code``` is available on the ```keydown``` events in Electron, which I use now. I'm not able to rewrite my code to use ```before-input-event```, for now.
atom/common/native_mate_converters/blink_converter.cc
v8::Isolate* isolate, v8::Local<v8::Value> val, blink::WebMouseEvent* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  if (!ConvertFromV8(isolate, val, static_cast<blink::WebInputEvent*>(out)))
    return false;
  if (!dict.Get("x", &out->x) || !dict.Get("y", &out->y))
    return false;
  if (!dict.Get("button", &out->button))
    out->button = blink::WebMouseEvent::Button::Left;
  dict.Get("globalX", &out->globalX);
  dict.Get("globalY", &out->globalY);
  dict.Get("movementX", &out->movementX);
  dict.Get("movementY", &out->movementY);
  dict.Get("clickCount", &out->clickCount);
  return true;
}
bool Converter<blink::WebMouseWheelEvent>::FromV8(