
### 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 an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
  * 6.1.5
* **Operating System:**
  * Windows 10
* **Last Known Working Electron version:**
  * ?

### Expected Behavior
If the window is focusable then the focus event should be triggered

### Actual Behavior
Setting the window initially to focusable: false and later to true again will not enable the focus events. Even when focusing the window, the event will never be triggered.

### To Reproduce
```
let browserWindow1 = new BrowserWindow({
	show: false,
	x: 1,
	y: 1,
	width: 800,
	height: 700,
	title: "test-title-1",
	focusable: false,
});
browserWindow1.on("focus", () => {console.log("focus");});
browserWindow1.loadURL("http://www.google.com");
browserWindow1.setFocusable(true);
browserWindow1.show();
```

shell/browser/native_window_views.cc
keyboard_event_handler_(new views::UnhandledKeyboardEventHandler) {
  options.Get(options::kTitle, &title_);
  bool menu_bar_autohide;
  if (options.Get(options::kAutoHideMenuBar, &menu_bar_autohide))
    root_view_->SetAutoHideMenuBar(menu_bar_autohide);
#if defined(OS_WIN)
  
  
  options.Get(options::kResizable, &resizable_);
  options.Get(options::kMinimizable, &minimizable_);
  options.Get(options::kMaximizable, &maximizable_);
  
  options.Get("thickFrame", &thick_frame_);
  if (transparent())
    thick_frame_ = false;
#endif
  if (enable_larger_than_screen())
    
    
    
    
    SetContentSizeConstraints(extensions::SizeConstraints(
        gfx::Size(), gfx::Size(INT_MAX / 10, INT_MAX / 10)));
  int width = 800, height = 600;
  options.Get(options::kWidth, &width);
  options.Get(options::kHeight, &height);
  gfx::Rect bounds(0, 0, width, height);
  widget_size_ = bounds.size();
  widget()->AddObserver(this);
  views::Widget::InitParams params;