- Electron version: 0.37.2
- Operating system: OS X 10.11.3 

https://github.com/atom/electron/pull/4700 added support for the `visibilitychange` event (thank you @zcbenz!), but that event doesn't make its way into `<webview>`s, which are always "visible" regardless of their host page's `document.visibilityState`.

Tested using the following setup

Host page:

``` coffee
console.log "<webview> host page initial visibilityState =", document.visibilityState

document.addEventListener "visibilitychange", ->
  console.log "<webview> host page visibilitychange event, visibilityState =", document.visibilityState
```

`<webview>`:

``` coffee
console.log "<webview> initial visibilityState =", document.visibilityState

addEventListener "visibilitychange", ->
  console.log "<webview> visibilitychange, visibilityState =", document.visibilityState

setInterval ->
  console.log "<webview> visibilityState =", document.visibilityState
, 3000
```

Host page logs:

```
<webview> host page initial visibilityState = visible
<webview> host page visibilitychange event, visibilityState = hidden
<webview> host page visibilitychange event, visibilityState = visible
```

`<webview>` logs:

```
<webview> initial visibilityState = visible
(24) <webview> visibilityState = visible
```

atom/browser/web_view_manager.h
public:
  WebViewManager();
  ~WebViewManager() override;
  void AddGuest(int guest_instance_id,
                int element_instance_id,
                content::WebContents* embedder,
                content::WebContents* web_contents);
  void RemoveGuest(int guest_instance_id);
 protected:
  
  content::WebContents* GetGuestByInstanceID(int owner_process_id,
                                             int element_instance_id) override;
  bool ForEachGuest(content::WebContents* embedder,
                    const GuestCallback& callback) override;
 private:
  struct WebContentsWithEmbedder {
    content::WebContents* web_contents;
    content::WebContents* embedder;
  };
  
  std::map<int, WebContentsWithEmbedder> web_contents_embedder_map_;
  struct ElementInstanceKey {
    int embedder_process_id;
    int element_instance_id;
    ElementInstanceKey(int embedder_process_id, int element_instance_id)