Reason for revert: Breaks downstream code. Original issue's description: > StartTimestamp generated randomly in RtpSender constructor > instead of not-randomly at SetSendingState(true) > Renamed to timestamp_offset_ to better match meaning of the variable. > > R=asapersson@webrtc.org, terelius@webrtc.org > > Committed: https://crrev.com/4466782ae43e1b1125a55ee7e18abd10dd37cede > Cr-Commit-Position: refs/heads/master@{#13796} TBR=asapersson@webrtc.org,terelius@webrtc.org # Skipping CQ checks because original CL landed less than 1 days ago. NOPRESUBMIT=true NOTREECHECKS=true NOTRY=true Review-Url: https://codereview.webrtc.org/2248413002 Cr-Commit-Position: refs/heads/master@{#13798}