* 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
guest_delegate_.reset(new WebViewGuestDelegate);
    params.guest_delegate = guest_delegate_.get();
    web_contents = content::WebContents::Create(params);
  } else if (IsOffScreen()) {
    bool transparent = false;
    options.Get("transparent", &transparent);
    content::WebContents::CreateParams params(session->browser_context());
    auto* view = new OffScreenWebContentsView(
        transparent, base::Bind(&WebContents::OnPaint, base::Unretained(this)));
    params.view = view;
    params.delegate_view = view;
    web_contents = content::WebContents::Create(params);
    view->SetWebContents(web_contents);
  } else {
    content::WebContents::CreateParams params(session->browser_context());
    web_contents = content::WebContents::Create(params);
  }
  InitWithSessionAndOptions(isolate, web_contents, session, options);
}
void WebContents::InitWithSessionAndOptions(v8::Isolate* isolate,
                                            content::WebContents *web_contents,
                                            mate::Handle<api::Session> session,
                                            const mate::Dictionary& options) {
  content::WebContentsObserver::Observe(web_contents);
  InitWithWebContents(web_contents, session->browser_context());
  managed_web_contents()->GetView()->SetDelegate(this);
  
  new WebContentsPreferences(web_contents, options);
  
  WebContentsPermissionHelper::CreateForWebContents(web_contents);