[GStreamer][MSE] video playback uses GstVA, except on YouTube

https://bugs.webkit.org/show_bug.cgi?id=261892

Reviewed by Xabier Rodriguez-Calvar.

Streams parsing is not needed for MediaStream cases because we do it upfront for incoming WebRTC
MediaStreams. It is however needed for MSE, otherwise decodebin3 might not auto-plug hardware
decoders.

* Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::MediaPlayerPrivateGStreamer::configureElement):

Canonical link: https://commits.webkit.org/269068@main

Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp

@@ -2234,9 +2234,11 @@ void MediaPlayerPrivateGStreamer::configureElement(GstElement* element)
     auto classifiers = elementClass.split('/');
 
     // In GStreamer 1.20 and older urisourcebin mishandles source elements with dynamic pads. This
-    // is not an issue in 1.22.
+    // is not an issue in 1.22. Streams parsing is not needed for MediaStream cases because we do it
+    // upfront for incoming WebRTC MediaStreams. It is however needed for MSE, otherwise decodebin3
+    // might not auto-plug hardware decoders.
     if (webkitGstCheckVersion(1, 22, 0) && g_str_has_prefix(elementName.get(), "urisourcebin") && (isMediaSource() || isMediaStreamPlayer()))
-        g_object_set(element, "use-buffering", FALSE, "parse-streams", FALSE, nullptr);
+        g_object_set(element, "use-buffering", FALSE, "parse-streams", !isMediaStreamPlayer(), nullptr);
 
     // In case of playbin3 with <video ... preload="auto">, instantiate
     // downloadbuffer element, otherwise the playbin3 would instantiate