### 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
gin_helper::Promise<gin_helper::Dictionary> promise(isolate);
  v8::Local<v8::Promise> handle = promise.GetHandle();
  auto* frame_host = ontents()->GetMainFrame();
  if (!frame_host) {
    promise.RejectWithErrorMessage("Failed to create memory dump");
    return handle;
  }
  auto pid = frame_host->GetProcess()->GetProcess().Pid();
  v8::Global<v8::Context> context(isolate, isolate->GetCurrentContext());
  memory_instrumentation::MemoryInstrumentation::GetInstance()
      ->RequestGlobalDumpForPid(
          pid, std::vector<std::string>(),
          base::BindOnce(&ElectronBindings::DidReceiveMemoryDump,
                         std::move(context), std::move(promise), pid));
  return handle;
}
v8::Local<v8::Promise> WebContents::TakeHeapSnapshot(
    v8::Isolate* isolate,
    const base::FilePath& file_path) {
  gin_helper::Promise<void> promise(isolate);
  v8::Local<v8::Promise> handle = promise.GetHandle();
  base::ThreadRestrictions::ScopedAllowIO allow_io;
  base::File file(file_path,
                  base::File::FLAG_CREATE_ALWAYS | base::File::FLAG_WRITE);
  if (!file.IsValid()) {
    promise.RejectWithErrorMessage("takeHeapSnapshot failed");
    return handle;
  }
  auto* frame_host = ontents()->GetMainFrame();
  if (!frame_host) {