* 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,
    content::NativeWebKeyboardEvent* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  if (!ConvertFromV8(isolate, val, static_cast<blink::WebKeyboardEvent*>(out)))
    return false;
  dict.Get("skipInBrowser", &out->skip_in_browser);
  return true;
}
v8::Local<v8::Value> Converter<content::NativeWebKeyboardEvent>::ToV8(
    v8::Isolate* isolate, const content::NativeWebKeyboardEvent& in) {
  mate::Dictionary dict = mate::Dictionary::CreateEmpty(isolate);
  if (in.type == blink::WebInputEvent::Type::RawKeyDown)
    dict.Set("type", "keyDown");
  else if (in.type == blink::WebInputEvent::Type::KeyUp)
    dict.Set("type", "keyUp");
  dict.Set("key", ui::KeycodeConverter::DomKeyToKeyString(in.domKey));
  using Modifiers = blink::WebInputEvent::Modifiers;
  dict.Set("isAutoRepeat", (in.modifiers & Modifiers::IsAutoRepeat) != 0);
  dict.Set("shift", (in.modifiers & Modifiers::ShiftKey) != 0);
  dict.Set("control", (in.modifiers & Modifiers::ControlKey) != 0);
  dict.Set("alt", (in.modifiers & Modifiers::AltKey) != 0);
  dict.Set("meta", (in.modifiers & Modifiers::MetaKey) != 0);
  return dict.GetHandle();
}
bool Converter<blink::WebMouseEvent>::FromV8(