
### 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
#include "ui/views/widget/desktop_aura/desktop_native_widget_aura.h"
#endif
namespace electron {
namespace {
#if defined(OS_WIN)
const LPCWSTR kUniqueTaskBarClassName = L"Shell_TrayWnd";
void FlipWindowStyle(HWND handle, bool on, DWORD flag) {
  DWORD style = ::GetWindowLong(handle, GWL_STYLE);
  if (on)
    style |= flag;
  else
    style &= ~flag;
  ::SetWindowLong(handle, GWL_STYLE, style);
}
gfx::Rect ScreenToDIPRect(HWND hwnd, const gfx::Rect& pixel_bounds) {
  float scale_factor = display::win::ScreenWin::GetScaleFactorForHWND(hwnd);
  gfx::Rect dip_rect = ScaleToRoundedRect(pixel_bounds, 1.0f / scale_factor);
  dip_rect.set_origin(
      display::win::ScreenWin::ScreenToDIPRect(hwnd, pixel_bounds).origin());
  return dip_rect;
}
gfx::Rect DIPToScreenRect(HWND hwnd, const gfx::Rect& pixel_bounds) {
  float scale_factor = display::win::ScreenWin::GetScaleFactorForHWND(hwnd);
  gfx::Rect screen_rect = ScaleToRoundedRect(pixel_bounds, scale_factor);
  screen_rect.set_origin(
      display::win::ScreenWin::DIPToScreenRect(hwnd, pixel_bounds).origin());
  return screen_rect;
}
#endif
class NativeWindowClientView : public views::ClientView {