
### 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
wm::SetShadowElevation(GetNativeWindow(),
                         has_shadow ? wm::kShadowElevationInactiveWindow
                                    : wm::kShadowElevationNone);
}
bool NativeWindowViews::HasShadow() {
  return GetNativeWindow()->GetProperty(wm::kShadowElevationKey) !=
         wm::kShadowElevationNone;
}
void NativeWindowViews::SetOpacity(const double opacity) {
#if defined(OS_WIN)
  const double boundedOpacity = base::ClampToRange(opacity, 0.0, 1.0);
  HWND hwnd = GetAcceleratedWidget();
  if (!layered_) {
    LONG ex_style = ::GetWindowLong(hwnd, GWL_EXSTYLE);
    ex_style |= WS_EX_LAYERED;
    ::SetWindowLong(hwnd, GWL_EXSTYLE, ex_style);
    layered_ = true;
  }
  ::SetLayeredWindowAttributes(hwnd, 0, boundedOpacity * 255, LWA_ALPHA);
  opacity_ = boundedOpacity;
#else
  opacity_ = 1.0;  
#endif
}
double NativeWindowViews::GetOpacity() {