[MSE] Use detach method for HTMLMediaElement::m_mediasource

https://bugs.webkit.org/show_bug.cgi?id=243796
<rdar://98471033>

Reviewed by Eric Carlson.

We should be using the `detachMediaSource` method in `setSrcObject` in
order to be consistent with the rest of the `HTMLMediaElement` implementation.

* Source/WebCore/html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::setSrcObject):

Canonical link: https://commits.webkit.org/253314@main

Author: youssefsoli

Source/WebCore/html/HTMLMediaElement.cpp

@@ -1122,7 +1122,7 @@ void HTMLMediaElement::setSrcObject(MediaProvider&& mediaProvider)
     m_mediaStreamSrcObject = nullptr;
 #endif
 #if ENABLE(MEDIA_SOURCE)
-    m_mediaSource = nullptr;
+    detachMediaSource();
 #endif
     m_blob = nullptr;