* Electron version: 1.6.3
* Operating system: Linux

### Expected behavior

When registering a `beforeunload` handler which cancels unloading, I would expect the handler to be called for every `BrowserWindow.reload()`.

### Actual behavior

On 1.6.3, the `beforeunload` handler is called for the first `reload` but not on subsequent calls. Even after removing the event handler, `reload` will not reload the window anymore.

### How to reproduce

```javascript
const cancelUnload = e => {
   console.log('cancelUnload called')
   e.returnValue = false
}

window.addEventListener('beforeunload', cancelUnload)

remote.getCurrentWindow().reload()  // [1] -> cancelUnload called
remote.getCurrentWindow().reload()  // [2] -> cancelUnload called

remote.getCurrentWindow().close()   // [3] -> cancelUnload called
remote.getCurrentWindow().close()   // [4] -> cancelUnload called

window.removeEventListener('beforeunload', cancelUnload)

remote.getCurrentWindow().reload()  // [5] -> reloads window

remote.getCurrentWindow().close()   // [6] -> closes window
```
I would expect `cancelUnload` to be printed to the console four times and the step at `[5]` to actually reload the window. This is works up to Electron 1.6.2.

On Electron 1.6.3 `cancelUnload` is not printed at step `[2]` (nor on any subsequent calls to reload). What's more, event after removing the `beforeunload` handler, `reload` does not work anymore: at step `[5]` the window does not reload (but closing the window at `[6]` works).
atom/browser/api/atom_api_web_contents.cc
blink::WebMouseWheelEvent mouse_wheel_event;
    if (mate::ConvertFromV8(isolate, input_event, &mouse_wheel_event)) {
      host->ForwardWheelEvent(mouse_wheel_event);
      return;
    }
  }
  isolate->ThrowException(v8::Exception::Error(mate::StringToV8(
      isolate, "Invalid event object")));
}
void WebContents::BeginFrameSubscription(mate::Arguments* args) {
  bool only_dirty = false;
  FrameSubscriber::FrameCaptureCallback callback;
  args->GetNext(&only_dirty);
  if (!args->GetNext(&callback)) {
    args->ThrowError();
    return;
  }
  const auto view = web_contents()->GetRenderWidgetHostView();
  if (view) {
    std::unique_ptr<FrameSubscriber> frame_subscriber(new FrameSubscriber(
        isolate(), view, callback, only_dirty));
    view->BeginFrameSubscription(std::move(frame_subscriber));
  }
}
void WebContents::EndFrameSubscription() {
  const auto view = web_contents()->GetRenderWidgetHostView();
  if (view)
    view->EndFrameSubscription();
}
void WebContents::StartDrag(const mate::Dictionary& item,