
### 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
widget()->CenterWindow(GetSize());
}
void NativeWindowViews::Invalidate() {
  widget()->SchedulePaintInRect(gfx::Rect(GetBounds().size()));
}
void NativeWindowViews::SetTitle(const std::string& title) {
  title_ = title;
  widget()->UpdateWindowTitle();
}
std::string NativeWindowViews::GetTitle() {
  return title_;
}
void NativeWindowViews::FlashFrame(bool flash) {
#if defined(OS_WIN)
  
  if (!flash) {
    FLASHWINFO fwi;
    fwi.cbSize = sizeof(fwi);
    fwi.hwnd = GetAcceleratedWidget();
    fwi.dwFlags = FLASHW_STOP;
    fwi.uCount = 0;
    FlashWindowEx(&fwi);
    return;
  }
#endif
  widget()->FlashFrame(flash);
}
void NativeWindowViews::SetSkipTaskbar(bool skip) {