LibWeb: Stop the video decoder thread when the video element is GC'd

Otherwise, the thread will continue to run and access the media data
buffer, which will have been freed.

The test here is a bit strange, but the issue would only consistently
repro after several GC runs.

Userland/Libraries/LibWeb/HTML/HTMLVideoElement.h

@@ -46,6 +46,7 @@ private:
     HTMLVideoElement(DOM::Document&, DOM::QualifiedName);
 
     virtual void initialize(JS::Realm&) override;
+    virtual void finalize() override;
     virtual void visit_edges(Cell::Visitor&) override;
 
     virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;