Change name of a NetEq internal member variable

In the StatisticsCalculator class, the member last_report_timestamp_
was unfortunately named. It's now been changed to
timestamps_since_last_report_, which describes it more accurately.

R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/25819004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7393 4adac7df-926f-26a2-2b94-8c16560cd09d