### 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.cc
.SetMethod("setDevToolsWebContents", &WebContents::SetDevToolsWebContents)
      .SetMethod("getNativeView", &WebContents::GetNativeView)
      .SetMethod("incrementCapturerCount", &WebContents::IncrementCapturerCount)
      .SetMethod("decrementCapturerCount", &WebContents::DecrementCapturerCount)
      .SetMethod("isBeingCaptured", &WebContents::IsBeingCaptured)
      .SetMethod("setWebRTCIPHandlingPolicy",
                 &WebContents::SetWebRTCIPHandlingPolicy)
      .SetMethod("getMediaSourceId", &WebContents::GetMediaSourceID)
      .SetMethod("getWebRTCIPHandlingPolicy",
                 &WebContents::GetWebRTCIPHandlingPolicy)
      .SetMethod("takeHeapSnapshot", &WebContents::TakeHeapSnapshot)
      .SetMethod("setImageAnimationPolicy",
                 &WebContents::SetImageAnimationPolicy)
      .SetMethod("_getProcessMemoryInfo", &WebContents::GetProcessMemoryInfo)
      .SetProperty("id", &WebContents::ID)
      .SetProperty("session", &WebContents::Session)
      .SetProperty("hostWebContents", &WebContents::HostWebContents)
      .SetProperty("devToolsWebContents", &WebContents::DevToolsWebContents)
      .SetProperty("debugger", &WebContents::Debugger)
      .SetProperty("mainFrame", &WebContents::MainFrame)
      .Build();
}
const char* WebContents::GetTypeName() {
  return "WebContents";
}
ElectronBrowserContext* WebContents::GetBrowserContext() const {
  return static_cast<ElectronBrowserContext*>(
      ontents()->GetBrowserContext());
}
 static