Intended to enable unit testing of the pacer with a mock PacketQueue. Bug: webrtc:8422 Change-Id: I142386b2d91ad0d5ba8f3f9d876e67972c490de4 Reviewed-on: https://webrtc-review.googlesource.com/17300 Reviewed-by: Philip Eliasson <philipel@webrtc.org> Commit-Queue: Niels Moller <nisse@webrtc.org> Cr-Commit-Position: refs/heads/master@{#20498}