Redo is broken for Widgets

[ferahl]

Steps to reproduce

1. Go to ​http://sdk.ckeditor.com/samples/mediaembed.html
2. Double click the YouTube embed to edit the URL
3. Enter a different URL e.g

   https://www.youtube.com/watch?v=_W9ewqSjkKk
   

4. Undo then Redo -> works
5. Undo
6. Click off the editor (blur)
7. Click back on to the editor and try observe the Redo button is active - click it

Expected result

Clicking Redo changes the state to the latest one (the video change).

Actual result

Nothing happens.

N.B If you had more Undo states before this, then going back to them and then Redoing to the end will allow you to do the final Redo, but it is an empty state with only a selection change (from non-selected widget to selected widget)

N.B On my own widget it only breaks if when coming back into the editor I click on something other than the widget and click Redo. But it seems broken either way on the sample page.

N.B Tested to be broken on 4.4.8 as well

Other details (browser, OS, CKEditor version, installed plugins)

[Jakub Ś]

After some checking I have realised that this is a duplicate of #13800