LibWeb: Fetch source file when HTMLTrackElement src attribute changes

This commit begins to implement the track processing model. When the
`src` attribute is updated, we now fetch the given source file.
Currently, we always fire an `error` event once fetching is completed,
as we don't support processing the fetched data.

Libraries/LibWeb/HTML/HTMLMediaElement.h

@@ -147,6 +147,8 @@ public:
     };
     CachedLayoutBoxes& cached_layout_boxes(Badge<Painting::MediaPaintable>) const { return m_layout_boxes; }
 
+    CORSSettingAttribute crossorigin() const { return m_crossorigin; }
+
 protected:
     HTMLMediaElement(DOM::Document&, DOM::QualifiedName);