Hide NagOverlay after checkboxes are checked

The nag overlay could be seen in the background of the image description
textarea when the textarea was focused (tested on Andrid Browser 4.2 and
Chrome for Android). This fixes it.

Change-Id: I9a95104ad0f7fce58b4290b01e0d3e8c1870612f
Merge "Hide NagOverlay after checkboxes are checked"