
### 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
#if defined(OS_WIN)
  return minimizable_;
#elif defined(USE_X11)
  return true;
#endif
}
base::string16 NativeWindowViews::GetWindowTitle() const {
  return base::UTF8ToUTF16(title_);
}
views::View* NativeWindowViews::GetContentsView() {
  return root_view_.get();
}
bool NativeWindowViews::ShouldDescendIntoChildForEventHandling(
    gfx::NativeView child,
    const gfx::Point& location) {
  
  if (draggable_region() &&
      draggable_region()->contains(location.x(), location.y()))
    return false;
  
  if (!has_frame() && CanResize()) {
    FramelessView* frame =
        static_cast<FramelessView*>(widget()->non_client_view()->frame_view());
    return frame->ResizingBorderHitTest(location) == HTNOWHERE;
  }
  return true;
}
views::ClientView* NativeWindowViews::CreateClientView(views::Widget* widget) {