- 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/api/atom_api_web_view_manager.cc
#include "atom/browser/web_contents_preferences.h"
#include "atom/browser/web_view_manager.h"
#include "atom/common/native_mate_converters/content_converter.h"
#include "atom/common/native_mate_converters/value_converter.h"
#include "atom/common/node_includes.h"
#include "content/public/browser/browser_context.h"
#include "native_mate/dictionary.h"
using atom::WebContentsPreferences;
namespace {
atom::WebViewManager* GetWebViewManager(content::WebContents* web_contents) {
  auto context = web_contents->GetBrowserContext();
  if (context) {
    auto manager = context->GetGuestManager();
    return static_cast<atom::WebViewManager*>(manager);
  } else {
    return nullptr;
  }
}
void AddGuest(int guest_instance_id,