Unify worker and network threads in full stack tests

Unify worker and network threads by default in `webrtc_perf_tests` to eliminate the startup packet-loss race condition introduced in CL 472240.

By making worker and network threads refer to the same thread object, the receiver sink registration (`AddSink`) runs synchronously on the same thread. This avoids the race window where incoming packets fail demuxing on the network thread before the asynchronous `AddSink` task is processed.

Performance verification results (comparing separate vs unified threads):

- Pc_Simulcast_HD_High:
  * dropped_frames: 63 -> 34 (-46% improvement)
  * max_skipped: 0.048 -> 0.025 (-47% improvement)
  * actual_encode_bitrate: 2011.686 kbps -> 2071.463 kbps (+3% recovery)
  * retransmission_bitrate: 0.134 kbps -> 0.000 kbps (-100% fully eliminated)

- Pc_Screenshare_Slides_Simulcast_No_Conference_Mode:
  * dropped_frames: 2 -> 1 (-50% improvement)
  * max_skipped: 0.045 -> 0.022 (-50% improvement)
  * actual_encode_bitrate: 240.117 kbps -> 352.958 kbps (+47% recovery)
  * transmission_bitrate: 371.318 kbps -> 520.972 kbps (+40% recovery)

- Pc_Simulcast_Vp8_3sl_High:
  * dropped_frames: 39 -> 27 (-30% improvement)
  * max_skipped: 0.029 -> 0.020 (-31% improvement)

Bug: webrtc:514760674
Change-Id: Ie6ad4542ac30a365af464854df383f0f14a03ed2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/474120
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Markus Handell <handellm@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47772}