### 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
const base::FilePath& path) {
  auto* isolated_context = storage::IsolatedContext::GetInstance();
  std::string root_name(kRootName);
  storage::IsolatedContext::ScopedFSHandle file_system =
      isolated_context->RegisterFileSystemForPath(
          storage::kFileSystemTypeLocal, std::string(), path, &root_name);
  content::ChildProcessSecurityPolicy* policy =
      content::ChildProcessSecurityPolicy::GetInstance();
  content::RenderViewHost* render_view_host = ontents->GetRenderViewHost();
  int renderer_id = render_view_host->GetProcess()->GetID();
  policy->GrantReadFileSystem(renderer_id, file_system.id());
  policy->GrantWriteFileSystem(renderer_id, file_system.id());
  policy->GrantCreateFileForFileSystem(renderer_id, file_system.id());
  policy->GrantDeleteFromFileSystem(renderer_id, file_system.id());
  if (!policy->CanReadFile(renderer_id, path))
    policy->GrantReadFile(renderer_id, path);
  return file_system.id();
}
FileSystem CreateFileSystemStruct(content::WebContents* ontents,