Only use BBRs pacer if the unit test is running BBR.
Otherwise use WebRTCs default pacer.

BUG=None

Review-Url: https://codereview.webrtc.org/3009363002
Cr-Original-Commit-Position: refs/heads/master@{#19797}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 661d94996b0301cee460d0e486e8371a05977aca