[RTCLogging] Enable RTCDiagnosticLogging by default

Enable the RTCDiagnosticLogging runtime-enabled feature by default
(status: "stable"), exposing the diagnostic logging static methods on
RTCPeerConnection.

Move the legacy navigator.rtc API (Navigator.rtc and the RTC interface)
behind a new experimental runtime-enabled feature flag,
RTCLegacyDiagnosticLogging, while keeping it enabled by the
RTCDiagnosticLogging origin trial.

I2S: https://groups.google.com/a/chromium.org/g/blink-dev/c/HeBQEx0dFaY
Spec: https://w3c.github.io/webrtc-extensions/#diagnostic-logging

Bug: 567470112
Change-Id: I66e19187650d38ee3a8979a4d1e5643c9f54bfcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8486546
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1708708}