Fix EZP-21711: Editor scrollbar disappears after zooming in & out in full screen

JIRA: https://jira.ez.no/browse/EZP-21711

# Description

If the user zooms in and then zooms out in Online Editor 5.5 (eZ Publish 4.7) in fullscreen mode, the vertical scrollbar disappears. This is a TinyMCE bug that was fixed in 3.5.5 so it does not appear in eZ Publish 5.x (5.0 comes with TinyMCE 3.5.7). 

This patch is just a backport of [the official fix](https://github.com/tinymce/tinymce/commit/59f009d48f0aef3a1e3caad50404f0374d55ee20).

# Tests

manual tests in IE8, IE9 and IE10, Firefox 24 and Chrome 30.