Fix moving over an image with the arrow keys in Firefox

There was a bug when you moved over an image with the arrow keys:
if your selection was on an image and you pressed an arrow key, the
selection would move but focus would remain with the paste target
rather than going back to the document node, which caused strange
symptoms (immobilizing the arrow keys and scrolling horizontally)
in Firefox.

Bug: 57600
Change-Id: Iaf6a49787dd2fd2f3f88abd0d1f5ae512fd3fd68
Merge "Fix moving over an image with the arrow keys in Firefox"