### 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
void Paste();
  void PasteAndMatchStyle();
  void Delete();
  void SelectAll();
  void Unselect();
  void Replace(const std::u16string& word);
  void ReplaceMisspelling(const std::u16string& word);
  uint32_t FindInPage(gin::Arguments* args);
  void StopFindInPage(content::StopFindAction action);
  void ShowDefinitionForSelection();
  void CopyImageAt(int x, int y);
  
  void Focus();
  bool IsFocused() const;
  
  void SendInputEvent(v8::Isolate* isolate, v8::Local<v8::Value> input_event);
  
  void BeginFrameSubscription(gin::Arguments* args);
  void EndFrameSubscription();
  
  void StartDrag(const gin_helper::Dictionary& item, gin::Arguments* args);
  
  
  v8::Local<v8::Promise> CapturePage(gin::Arguments* args);
  
  bool IsGuest() const;
  void AttachToIframe(content::WebContents* embedder_web_contents,
                      int embedder_frame_id);
  void DetachFromOuterFrame();