### 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/common_web_contents_delegate.cc
int request_id,
    const std::string& file_system_path,
    const std::string& excluded_folders_message) {
  if (!IsDevToolsFileSystemAdded(GetDevToolsWebContents(), file_system_path)) {
    OnDevToolsIndexingDone(request_id, file_system_path);
    return;
  }
  if (devtools_indexing_jobs_.count(request_id) != 0)
    return;
  std::vector<std::string> excluded_folders;
  std::unique_ptr<base::Value> parsed_excluded_folders =
      base::JSONReader::ReadDeprecated(excluded_folders_message);
  if (parsed_excluded_folders && parsed_excluded_folders->is_list()) {
    const std::vector<base::Value>& folder_paths =
        parsed_excluded_folders->GetList();
    for (const base::Value& folder_path : folder_paths) {
      if (folder_path.is_string())
        excluded_folders.push_back(folder_path.GetString());
    }
  }
  devtools_indexing_jobs_[request_id] =
      scoped_refptr<DevToolsFileSystemIndexer::FileSystemIndexingJob>(
          devtools_file_system_indexer_->IndexPath(
              file_system_path, excluded_folders,
              base::BindRepeating(
                  &CommonWebContentsDelegate::OnDevToolsIndexingWorkCalculated,
                  weak_factory_.GetWeakPtr(), request_id, file_system_path),