
### 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
return;
    }
  }
#endif
  
  gfx::Size content_size = GetContentSize();
  bool should_reset_size = use_content_size_ && has_frame() &&
                           !IsMenuBarAutoHide() &&
                           ((!!menu_model) != root_view_->HasMenu());
  root_view_->SetMenu(menu_model);
  if (should_reset_size) {
    
    int menu_bar_height = root_view_->GetMenuBarHeight();
    extensions::SizeConstraints constraints = GetContentSizeConstraints();
    if (constraints.HasMinimumSize()) {
      gfx::Size min_size = constraints.GetMinimumSize();
      min_size.set_height(min_size.height() + menu_bar_height);
      constraints.set_minimum_size(min_size);
    }
    if (constraints.HasMaximumSize()) {
      gfx::Size max_size = constraints.GetMaximumSize();
      max_size.set_height(max_size.height() + menu_bar_height);
      constraints.set_maximum_size(max_size);
    }
    SetContentSizeConstraints(constraints);
    
    SetContentSize(content_size);
  }
}
void NativeWindowViews::AddBrowserView(NativeBrowserView* view) {