* 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
Emit("will-destroy");
  ResetManagedWebContents(async);
}
bool WebContents::DidAddMessageToConsole(content::WebContents* source,
                                         int32_t level,
                                         const base::string16& message,
                                         int32_t line_no,
                                         const base::string16& source_id) {
  if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN) {
    return false;
  } else {
    Emit("console-message", level, message, line_no, source_id);
    return true;
  }
}
void WebContents::OnCreateWindow(
    const GURL& target_url,
    const std::string& frame_name,
    WindowOpenDisposition disposition,
    const std::vector<std::string>& features,
    const scoped_refptr<content::ResourceRequestBodyImpl>& body) {
  if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN)
    Emit("-new-window", target_url, frame_name, disposition, features, body);
  else
    Emit("new-window", target_url, frame_name, disposition, features);
}
void WebContents::WebContentsCreated(content::WebContents* source_contents,