
### 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
#include "ui/gfx/native_widget_types.h"
#include "ui/views/background.h"
#include "ui/views/controls/webview/unhandled_keyboard_event_handler.h"
#include "ui/views/controls/webview/webview.h"
#include "ui/views/widget/native_widget_private.h"
#include "ui/views/widget/widget.h"
#include "ui/views/window/client_view.h"
#include "ui/wm/core/shadow_types.h"
#include "ui/wm/core/window_util.h"
#if defined(USE_X11)
#include "base/strings/string_util.h"
#include "shell/browser/browser.h"
#include "shell/browser/linux/unity_service.h"
#include "shell/browser/ui/views/frameless_view.h"
#include "shell/browser/ui/views/global_menu_bar_x11.h"
#include "shell/browser/ui/views/native_frame_view.h"
#include "shell/browser/ui/x/event_disabler.h"
#include "shell/browser/ui/x/window_state_watcher.h"
#include "shell/browser/ui/x/x_window_utils.h"
#include "ui/base/x/x11_util.h"
#include "ui/gfx/x/x11_types.h"
#include "ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h"
#include "ui/views/window/native_frame_view.h"
#elif defined(OS_WIN)
#include "shell/browser/ui/views/win_frame_view.h"
#include "shell/browser/ui/win/atom_desktop_native_widget_aura.h"
#include "skia/ext/skia_utils_win.h"
#include "ui/base/win/shell.h"
#include "ui/display/screen.h"
#include "ui/display/win/screen_win.h"