Fix potential deadlock in RTCStatsCollector during shutdown.

When stats gathering on the network thread is cancelled (e.g., during PC
closure), the `SafeTask` evaluating the partial results on the network
thread may be dropped without executing. This causes its closures
(including `RtpTransceiverStatsInfo` objects) to be destructed on the
network thread.

Because `RtpTransceiverStatsInfo` holds `scoped_refptr` references to
`RtpTransceiver` and `RtpReceiverInternal` (which may respectively hold
`VideoTrackProxyWithInternal`), their destruction on the network thread
can trigger a blocking call back to the signaling thread (via the
proxies), potentially leading to a deadlock if the signaling thread is
issuing a blocking call to the network thread.

This change prevents clears the object references (`transceiver` and
`receivers`) on the signaling thread before posting to the network
thread. When the task is dropped on the network thread, the subsequent
destruction of `RtpTransceiverStatsInfo` will be a no-op that avoids
cross-thread blocking.

Bug: b/483026914
Change-Id: Ifc5c317e6a26cc0c97c9ca67101ef8b0cc90ba1e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/448940
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46897}