* 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.h
void OnEnterFullscreenModeForTab(content::WebContents* source,
                                   const GURL& origin,
                                   bool allowed);
  
  void OnCreateWindow(
      const GURL& target_url,
      const std::string& frame_name,
      WindowOpenDisposition disposition,
      const std::vector<std::string>& features,
      const scoped_refptr<content::ResourceRequestBodyImpl>& body);
  
  v8::Local<v8::Value> GetWebPreferences(v8::Isolate* isolate);
  
  v8::Local<v8::Value> GetOwnerBrowserWindow();
  
  int32_t ID() const;
  v8::Local<v8::Value> Session(v8::Isolate* isolate);
  content::WebContents* HostWebContents();
  v8::Local<v8::Value> DevToolsWebContents(v8::Isolate* isolate);
  v8::Local<v8::Value> Debugger(v8::Isolate* isolate);
  WebContentsZoomController* GetZoomController() { return zoom_controller_; }
 protected:
  WebContents(v8::Isolate* isolate,
              content::WebContents* web_contents,
              Type type);
  WebContents(v8::Isolate* isolate, const mate::Dictionary& options);
  ~WebContents();
  void InitWithSessionAndOptions(v8::Isolate* isolate,
                                 content::WebContents *web_contents,
                                 mate::Handle<class Session> session,