Revert conversion from TickTime to int64_t in paced sender.

Introduced with r6600, causing flakes in SuspendBelowMinBitrate. The reason for this flake is currently unknown.

R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17879004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6605 4adac7df-926f-26a2-2b94-8c16560cd09d