### 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
.ThrowError("webContents.print(): Invalid print settings specified.");
    return;
  }
  printing::CompletionCallback callback;
  if (args->Length() == 2 && !args->GetNext(&callback)) {
    gin_helper::ErrorThrower(args->isolate())
        .ThrowError("webContents.print(): Invalid optional callback provided.");
    return;
  }
   Set optional silent printing
  bool silent = false;
  options.Get("silent", &silent);
  bool print_background = false;
  options.Get("printBackground", &print_background);
  settings.SetBoolKey(printing::kSettingShouldPrintBackgrounds,
                      print_background);
   Set custom margin settings
  gin_helper::Dictionary margins =
      gin::Dictionary::CreateEmpty(args->isolate());
  if (options.Get("margins", &margins)) {
    printing::mojom::MarginType margin_type =
        printing::mojom::MarginType::kDefaultMargins;
    margins.Get("marginType", &margin_type);
    settings.SetIntKey(printing::kSettingMarginsType,
                       static_cast<int>(margin_type));
    if (margin_type == printing::mojom::MarginType::kCustomMargins) {
      base::Value custom_margins(base::Value::Type::DICTIONARY);
      int top = 0;
      margins.Get("top", &top);
      custom_margins.SetIntKey(printing::kSettingMarginTop, top);