* 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
else if (type == "touchstart")
      *out = blink::WebInputEvent::TouchStart;
    else if (type == "touchmove")
      *out = blink::WebInputEvent::TouchMove;
    else if (type == "touchend")
      *out = blink::WebInputEvent::TouchEnd;
    else if (type == "touchcancel")
      *out = blink::WebInputEvent::TouchCancel;
    return true;
  }
};
template<>
struct Converter<blink::WebMouseEvent::Button> {
  static bool FromV8(v8::Isolate* isolate, v8::Handle<v8::Value> val,
                     blink::WebMouseEvent::Button* out) {
    std::string button = base::ToLowerASCII(V8ToString(val));
    if (button == "left")
      *out = blink::WebMouseEvent::Button::Left;
    else if (button == "middle")
      *out = blink::WebMouseEvent::Button::Middle;
    else if (button == "right")
      *out = blink::WebMouseEvent::Button::Right;
    else
      return false;
    return true;
  }
};
template<>
struct Converter<blink::WebInputEvent::Modifiers> {
  static bool FromV8(v8::Isolate* isolate, v8::Handle<v8::Value> val,