Add unit to dropped frames.

Missing unit causes less dropped frames to be reported as a regression
and not an improvement.

R=stefan@webrtc.org
BUG=chromium:429206

Review URL: https://webrtc-codereview.appspot.com/25139004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7666 4adac7df-926f-26a2-2b94-8c16560cd09d