Change MockRtpRtcp to NiceMock<MockRtpRtcp> throughout PacketRouterTest and PacketRouterRembTest (12 tests in total), to suppress a large number of warnings which are currently ignored.
BUG=None
Review-Url: https://codereview.webrtc.org/2977533002
Cr-Commit-Position: refs/heads/master@{#18946}