* 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
v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  devtools_web_contents_.Reset();
  Emit("devtools-closed");
}
bool WebContents::OnMessageReceived(const IPC::Message& message) {
  bool handled = true;
  IPC_BEGIN_MESSAGE_MAP(WebContents, message)
    IPC_MESSAGE_HANDLER(AtomViewHostMsg_Message, OnRendererMessage)
    IPC_MESSAGE_HANDLER_DELAY_REPLY(AtomViewHostMsg_Message_Sync,
                                    OnRendererMessageSync)
    IPC_MESSAGE_HANDLER_DELAY_REPLY(AtomViewHostMsg_SetTemporaryZoomLevel,
                                    OnSetTemporaryZoomLevel)
    IPC_MESSAGE_HANDLER_DELAY_REPLY(AtomViewHostMsg_GetZoomLevel,
                                    OnGetZoomLevel)
    IPC_MESSAGE_HANDLER_CODE(ViewHostMsg_SetCursor, OnCursorChange,
      handled = false)
    IPC_MESSAGE_UNHANDLED(handled = false)
  IPC_END_MESSAGE_MAP()
  return handled;
}
void WebContents::WebContentsDestroyed() {