Makes construction simpler, and allows the ts_extrapolator_ pointer to be marked const. Followup to https://webrtc-review.googlesource.com/c/src/+/190721 Bug: webrtc:12102 Change-Id: I2abeb960935b5470509f654a4a9d5121c8001900 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/190981 Reviewed-by: Philip Eliasson <philipel@webrtc.org> Commit-Queue: Niels Moller <nisse@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32535}