
### 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
NotifyWindowMove();
}
void NativeWindowViews::HandleKeyboardEvent(
    content::WebContents*,
    const content::NativeWebKeyboardEvent& event) {
#if defined(OS_LINUX)
  if (event.windows_key_code == ui::VKEY_BROWSER_BACK)
    NotifyWindowExecuteAppCommand(kBrowserBackward);
  else if (event.windows_key_code == ui::VKEY_BROWSER_FORWARD)
    NotifyWindowExecuteAppCommand(kBrowserForward);
#endif
  keyboard_event_handler_->HandleKeyboardEvent(event,
                                               root_view_->GetFocusManager());
  root_view_->HandleKeyEvent(event);
}
#if defined(OS_LINUX)
void NativeWindowViews::OnMouseEvent(ui::MouseEvent* event) {
  if (event->type() != ui::ET_MOUSE_PRESSED)
    return;
  if (event->changed_button_flags() == ui::EF_BACK_MOUSE_BUTTON)
    NotifyWindowExecuteAppCommand(kBrowserBackward);
  else if (event->changed_button_flags() == ui::EF_FORWARD_MOUSE_BUTTON)
    NotifyWindowExecuteAppCommand(kBrowserForward);
}
#endif
ui::WindowShowState NativeWindowViews::GetRestoredState() {