
### 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
const gfx::AcceleratedWidget accelerated_widget = GetAcceleratedWidget();
  content::DesktopMediaID::Id window_handle = content::DesktopMediaID::kNullId;
  content::DesktopMediaID::Id aura_id = content::DesktopMediaID::kNullId;
#if defined(OS_WIN)
  window_handle =
      reinterpret_cast<content::DesktopMediaID::Id>(accelerated_widget);
#elif defined(USE_X11)
  window_handle = accelerated_widget;
#endif
  aura::WindowTreeHost* const host =
      aura::WindowTreeHost::GetForAcceleratedWidget(accelerated_widget);
  aura::Window* const aura_window = host ? host->window() : nullptr;
  if (aura_window) {
    aura_id = content::DesktopMediaID::RegisterNativeWindow(
                  content::DesktopMediaID::TYPE_WINDOW, aura_window)
                  .window_id;
  }
  
  
  content::DesktopMediaID result = content::DesktopMediaID(
      content::DesktopMediaID::TYPE_WINDOW, window_handle);
  
  
  
  result.window_id = aura_id;
  return result;
}
gfx::AcceleratedWidget NativeWindowViews::GetAcceleratedWidget() const {