Only one implementation of MockRtpPacketSink once

MockRtpPacketSink has three identical implementations now, so time to move it to its own file.

BUG=None

Review-Url: https://codereview.webrtc.org/2988853002
Cr-Original-Commit-Position: refs/heads/master@{#19183}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e2173d9f0d2d991d576ffa71a18761af69b72ede
5 files changed