The source node is typically a text node, and it may be editable by
`contenteditable` or `designMode`. In that case, the source node may be removed
during the DnD session even without tricky JS code. In this case, Blink and
WebKit updates the source node to dispatch `dragend` to the editing host.
This behavior does not conform to the standardized DnD behavior, however,
this is reasonable for editor apps which want to listen to events in editing
host or window/document for footprint and/or performance reason. Therefore,
we should follow their behavior.
Differential Revision: https://phabricator.services.mozilla.com/D172091
e5f024930b