Release RTCRtpTransceiver.stop

The chromium feature is https://www.chromestatus.com/feature/5410592384876544
The intent-to-implement-and-ship thread is
https://groups.google.com/a/chromium.org/g/blink-dev/c/dAeG7K7HPG8

The feature is covered by WPT tests.

Bug: chromium:980879
Change-Id: Icac0d2f4b116bc775a9635c7441f6f8cbad371a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438065Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813012}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1637,7 +1637,7 @@
     // Enables the use of |RTCRtpTransceiver::stop()|
     {
       name: "RTCRtpTransceiverStop",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "RTCStatsRelativePacketArrivalDelay",

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -5583,6 +5583,7 @@ interface RTCRtpTransceiver
     getter stopped
     method constructor
     method setCodecPreferences
+    method stop
     setter direction
 interface RTCSctpTransport : EventTarget
     attribute @@toStringTag