Change type of nack_last_time_sent_full_ from uint32_t to int64_t.
Could cause nack requests to be sent too frequently.

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/27339004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7825 4adac7df-926f-26a2-2b94-8c16560cd09d