This change moves the responsibility of posting
EncoderSwitchRequestCallback calls closer to the top-level
users which has a better idea about threading requirements.
The change is planned to be followed-up with more changes removing
the need for VSE to post to the worker thread.
Bug: webrtc:13414, chromium:1255737
Change-Id: I57a2962a70e9f245460c59c0d61824371394b952
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/238420
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35387}
d7eef66a39