Bind default sink to existing unsignaled receive stream

WebRtcVideoReceiveChannel::SetDefaultSink() previously only updated
the cached default_sink_ pointer. If the first RTP packet for an
unsignaled SSRC arrived before the application attached a sink
(e.g. the remote track exists but AddSink had not yet been called),
the default receive stream was created with a null sink, and the
later SetDefaultSink call had no way to reach it. Frames were then
silently dropped until the unsignaled SSRC changed and the stream
was recreated.

Propagate the new sink to the already-existing unsignaled receive
stream, matching the behavior of WebRtcVoiceReceiveChannel::
SetDefaultRawAudioSink().

Bug: None
Change-Id: I2c0e90c054b7118e752ca637dab709b5b97440aa
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/475840
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47823}