[Webkit] Error thrown in while right-clicking a nested editable

[Olek Nowodziński]

1. Open plugins/image2/samples/image2.html
2. Select a widget.
3. C-c.
4. "New page"
5. C-v.
6. Right click nested editable.

Expected: Context menu to be displayed.

Actual: Uncaught TypeError: Cannot call method 'checkReadOnly' of undefined plugin.js:382 (clipboard)

Funny facts:

• It's not reproducible in standard sample, e.g. with rich contents, many widgets (ignoring steps 2-5).
• It's not reproducible in such case:
  1. Follow steps until 5.
  2. Right click the image (but not a nested editable).
  3. Proceed with 6.
  4. No error thrown, contextmenu appears.
• Does it have anything in common with #10433?