Reduce the number of webrtc:: prefixes in code

This is achieved by moving most anonymous namespaces inside the
webrtc namespace and modifying as appropriate.

Bug: None
Change-Id: Iff299c45f92ae5aaa736722ed6630df9ad5525bf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/454240
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47077}