### Preflight Checklist
<!-- Please ensure you've completed the following steps by replacing [ ] with [x]-->

* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
5.0.0
* **Operating System:**
MacOS 10.13.6 / Windows 10
* **Last Known Working Electron version:**:
Electron v2.0.8

### Expected Behavior
Exiting fullscreen content ( ie. A Youtube Video displayed in a iframe ) should return the to app.
### Actual Behavior
Video size shrinks, but screen stays black , see https://cl.ly/f7af08d44619

### To Reproduce
main.js

```
const {app, BrowserWindow} = require('electron')
const path = require('path')
const url = require('url')

let window = null

// Wait until the app is ready
app.once('ready', () => {
  // Create a new window
  window = new BrowserWindow({
    // Set the initial width to 500px
    width: 600,
    // Set the initial height to 400px
    height: 600,
    // Don't show the window until it's ready, this prevents any white flickering
    show: false,
    webPreferences: {
      webSecurity: false,
      nodeIntegration: true,
      allowRunningInsecureContent: true,
    },
  })

  window.loadURL(url.format({
    pathname: path.join(__dirname, 'index.html'),
    protocol: 'file:',
    slashes: true
  }))

  window.once('ready-to-show', () => {
    window.show()
  })
})

```

index.html

```
<!DOCTYPE html>
<html>
</head>
<body>
  <div>
    <iframe width="560" height="315" src="https://www.youtube.com/embed/eBpxovL0zng" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen>
    </iframe>
  </div>
  </body>
</html>
```

### Screenshots
Screen recording https://cl.ly/f7af08d44619


atom/browser/api/atom_api_web_contents.cc
.SetMethod("_goForward", &WebContents::GoForward)
      .SetMethod("_goToOffset", &WebContents::GoToOffset)
      .SetMethod("isCrashed", &WebContents::IsCrashed)
      .SetMethod("setUserAgent", &WebContents::SetUserAgent)
      .SetMethod("getUserAgent", &WebContents::GetUserAgent)
      .SetMethod("savePage", &WebContents::SavePage)
      .SetMethod("openDevTools", &WebContents::OpenDevTools)
      .SetMethod("closeDevTools", &WebContents::CloseDevTools)
      .SetMethod("isDevToolsOpened", &WebContents::IsDevToolsOpened)
      .SetMethod("isDevToolsFocused", &WebContents::IsDevToolsFocused)
      .SetMethod("enableDeviceEmulation", &WebContents::EnableDeviceEmulation)
      .SetMethod("disableDeviceEmulation", &WebContents::DisableDeviceEmulation)
      .SetMethod("toggleDevTools", &WebContents::ToggleDevTools)
      .SetMethod("inspectElement", &WebContents::InspectElement)
      .SetMethod("setIgnoreMenuShortcuts", &WebContents::SetIgnoreMenuShortcuts)
      .SetMethod("setAudioMuted", &WebContents::SetAudioMuted)
      .SetMethod("isAudioMuted", &WebContents::IsAudioMuted)
      .SetMethod("isCurrentlyAudible", &WebContents::IsCurrentlyAudible)
      .SetMethod("undo", &WebContents::Undo)
      .SetMethod("redo", &WebContents::Redo)
      .SetMethod("cut", &WebContents::Cut)
      .SetMethod("copy", &WebContents::Copy)
      .SetMethod("paste", &WebContents::Paste)
      .SetMethod("pasteAndMatchStyle", &WebContents::PasteAndMatchStyle)
      .SetMethod("delete", &WebContents::Delete)
      .SetMethod("selectAll", &WebContents::SelectAll)
      .SetMethod("unselect", &WebContents::Unselect)
      .SetMethod("replace", &WebContents::Replace)
      .SetMethod("replaceMisspelling", &WebContents::ReplaceMisspelling)
      .SetMethod("findInPage", &WebContents::FindInPage)