Prevent warnings in PacketRouterTest and PacketRouterRembTest

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-Original-Commit-Position: refs/heads/master@{#18946}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6c9556e7f19bddbb38fadf22fd3b52672d0f59ed
1 file changed