Displays: Can't modify UI scale slide with keyboard

It should be possible to modify the UI Scale slider using the keyboard.

To reproduce:
1) Launch the Displays control panel
2) Click the thumb/handle in the UI scale slider once, to select it. Notice that the handle gets an orange border, to indicate that it's selected
3) Press the left/right arrow keys to modify the UI scale

Expected results:
The UI scale slider responds to the arrow keys

Actual results:
Nothing happens when you use the keyboard to manipulate the UI scale slider

This is an accessibility issue. It's also a problem if you set the UI scale so big that the UI scale widget is no longer drawn onscreen - you can't use keyboard navigation to reset it.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity-control-center 14.04.3+14.04.20140319-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-19.39-generic 3.13.6
Uname: Linux 3.13.0-19-generic x86_64
ApportVersion: 2.13.3-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Mar 24 17:40:33 2014
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-precise-amd64-20130203-1
EcryptfsInUse: Yes
InstallationDate: Installed on 2013-12-02 (111 days ago)
InstallationMedia: Ubuntu 12.04 "Precise" - Build amd64 LIVE Binary 20130203-13:50
SourcePackage: unity-control-center
UpgradeStatus: Upgraded to trusty on 2014-02-12 (40 days ago)
usr_lib_unity-control-center:
 activity-log-manager 0.9.7-0ubuntu10
 deja-dup 29.5-0ubuntu2
 gnome-control-center 1:3.6.3-0ubuntu54