Fix comments in common_types.h

Two of the metrics in NetworkStatistics were desribed as being in
percent, while they are in fact fractions between 0 and 1, scaled
to Q14 domain.

R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/30689004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7411 4adac7df-926f-26a2-2b94-8c16560cd09d