### 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
promise.RejectWithErrorMessage("takeHeapSnapshot failed");
    return handle;
  }
  if (!frame_host->IsRenderFrameCreated()) {
    promise.RejectWithErrorMessage("takeHeapSnapshot failed");
    return handle;
  }
   This dance with `base::Owned` is to ensure that the interface stays alive
   until the callback is called. Otherwise it would be closed at the end of
   this function.
  auto electron_renderer =
      std::make_unique<mojo::Remote<mojom::ElectronRenderer>>();
  frame_host->GetRemoteInterfaces()->GetInterface(
      electron_renderer->BindNewPipeAndPassReceiver());
  auto* raw_ptr = electron_renderer.get();
  (*raw_ptr)->TakeHeapSnapshot(
      mojo::WrapPlatformFile(base::ScopedPlatformFile(file.TakePlatformFile())),
      base::BindOnce(
          [](mojo::Remote<mojom::ElectronRenderer>* ep,
             gin_helper::Promise<void> promise, bool success) {
            if (success) {
              promise.Resolve();
            } else {
              promise.RejectWithErrorMessage("takeHeapSnapshot failed");
            }
          },
          base::Owned(std::move(electron_renderer)), std::move(promise)));
  return handle;
}
void WebContents::GrantDevicePermission(