Cherry-pick 272605@main (087cd69). https://bugs.webkit.org/show_bug.cgi?id=266758

    [MSE][GStreamer] Fix releasing Stream and GstStream objects
    https://bugs.webkit.org/show_bug.cgi?id=266758

    Reviewed by Philippe Normand.

    There was a circular reference between WebKitMediaSrcPadPrivate which
    owns a RefPtr<Stream>, and Stream which owns a GRefPtr<GstPad> const pad.

    * Source/WebCore/platform/graphics/gstreamer/mse/WebKitMediaSourceGStreamer.cpp:
    (webKitMediaSrcTearDownStream):

    Canonical link: https://commits.webkit.org/272605@main

Author: blino

Source/WebCore/platform/graphics/gstreamer/mse/WebKitMediaSourceGStreamer.cpp

@@ -347,8 +347,11 @@ static void webKitMediaSrcTearDownStream(WebKitMediaSrc* source, const AtomStrin
     // Stop the thread now.
     gst_pad_set_active(stream->pad.get(), false);
 
-    if (source->priv->isStarted())
-        gst_element_remove_pad(GST_ELEMENT(source), stream->pad.get());
+    if (source->priv->isStarted()) {
+        WebKitMediaSrcPad* pad = WEBKIT_MEDIA_SRC_PAD(stream->pad.get());
+        gst_element_remove_pad(GST_ELEMENT(source), GST_PAD(pad));
+        pad->priv->stream = nullptr;
+    }
     source->priv->streams.remove(name);
 }