Add "score" unit to SSIM perf score output.

Currently, the SSIM values don't have a unit, which makes
them default to lower being better rather than the opposite
(which is the case for SSIM).

R=phoglund@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/41709004

Cr-Commit-Position: refs/heads/master@{#8183}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8183 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed