### Preflight Checklist
* [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 a feature request that matches the one I want to file, without success.

### Problem Description
When working with BrowserViews, there's no way to listen to focus changes on either BrowserView itself or its WebContents.

### Proposed Solution
On BrowserWindow, there are `focus` and `blur` events. The same events need to be emitted on BrowserView/WebContents.
i.e. : 
```
browserView.on('focus', () => {
    // view is focused, do stuff
});
```
or  
```
browserView.webContents.on('focus', () => {
    // contents are focused, do stuff
});
```

### Alternatives Considered
I can't think of a good way of working around this lack of functionality
shell/browser/api/electron_api_web_contents.h
v8::Isolate* isolate,
      const gin_helper::Dictionary& web_preferences);
  
  static gin::WrapperInfo kWrapperInfo;
  static v8::Local<v8::ObjectTemplate> FillObjectTemplate(
      v8::Isolate*,
      v8::Local<v8::ObjectTemplate>);
  const char* GetTypeName() override;
  void Destroy();
  base::WeakPtr<WebContents> GetWeakPtr() { return weak_factory_.GetWeakPtr(); }
  bool GetBackgroundThrottling() const;
  void SetBackgroundThrottling(bool allowed);
  int GetProcessID() const;
  base::ProcessId GetOSProcessID() const;
  Type GetType() const;
  bool Equal(const WebContents* web_contents) const;
  void LoadURL(const GURL& url, const gin_helper::Dictionary& options);
  void Reload();
  void ReloadIgnoringCache();
  void DownloadURL(const GURL& url);
  GURL GetURL() const;
  std::u16string GetTitle() const;
  bool IsLoading() const;
  bool IsLoadingMainFrame() const;
  bool IsWaitingForResponse() const;
  void Stop();
  bool CanGoBack() const;
  void GoBack();
  bool CanGoForward() const;
  void GoForward();