### 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
auto file_system_paths = GetAddedFileSystemPaths(GetDevToolsWebContents());
  if (file_system_paths.empty()) {
    base::ListValue empty_file_system_value;
    inspectable_ontents_->CallClientFunction(
        "DevToolsAPI.fileSystemsLoaded", &empty_file_system_value, nullptr,
        nullptr);
    return;
  }
  std::vector<FileSystem> file_systems;
  for (const auto& file_system_path : file_system_paths) {
    base::FilePath path =
        base::FilePath::FromUTF8Unsafe(file_system_path.first);
    std::string file_system_id =
        RegisterFileSystem(GetDevToolsWebContents(), path);
    FileSystem file_system =
        CreateFileSystemStruct(GetDevToolsWebContents(), file_system_id,
                               file_system_path.first, file_system_path.second);
    file_systems.push_back(file_system);
  }
  base::ListValue file_system_value;
  for (const auto& file_system : file_systems)
    file_system_value.Append(CreateFileSystemValue(file_system));
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.fileSystemsLoaded", &file_system_value, nullptr, nullptr);
}
void WebContents::DevToolsAddFileSystem(