* 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::WebFloatPoint* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  return dict.Get("x", &out->x) && dict.Get("y", &out->y);
}
bool Converter<blink::WebPoint>::FromV8(
    v8::Isolate* isolate, v8::Local<v8::Value> val, blink::WebPoint* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  return dict.Get("x", &out->x) && dict.Get("y", &out->y);
}
bool Converter<blink::WebSize>::FromV8(
    v8::Isolate* isolate, v8::Local<v8::Value> val, blink::WebSize* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  return dict.Get("width", &out->width) && dict.Get("height", &out->height);
}
bool Converter<blink::WebDeviceEmulationParams>::FromV8(