### 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
std::u16string device_name;
  options.Get("deviceName", &device_name);
  if (!device_name.empty() && !IsDeviceNameValid(device_name)) {
    gin_helper::ErrorThrower(args->isolate())
        .ThrowError("webContents.print(): Invalid deviceName provided.");
    return;
  }
  int scale_factor = 100;
  options.Get("scaleFactor", &scale_factor);
  settings.SetIntKey(printing::kSettingScaleFactor, scale_factor);
  int pages_per_sheet = 1;
  options.Get("pagesPerSheet", &pages_per_sheet);
  settings.SetIntKey(printing::kSettingPagesPerSheet, pages_per_sheet);
   True if the user wants to print with collate.
  bool collate = true;
  options.Get("collate", &collate);
  settings.SetBoolKey(printing::kSettingCollate, collate);
   The number of individual copies to print
  int copies = 1;
  options.Get("copies", &copies);
  settings.SetIntKey(printing::kSettingCopies, copies);
   Strings to be printed as headers and footers if requested by the user.
  std::string header;
  options.Get("header", &header);
  std::string footer;
  options.Get("footer", &footer);
  if (!(header.empty() && footer.empty())) {
    settings.SetBoolKey(printing::kSettingHeaderFooterEnabled, true);
    settings.SetStringKey(printing::kSettingHeaderFooterTitle, header);
    settings.SetStringKey(printing::kSettingHeaderFooterURL, footer);