GetCurrentSession now needs to know the widget to return the drag
session in content processes. It now uses an explicitly provided
window or the entry global JS context (that must be a window).
This also adds GetDragSession to DOMWindowUtils, and fixes some small
bugs in EventUtils.
Original Revision: https://phabricator.services.mozilla.com/D211064
Differential Revision: https://phabricator.services.mozilla.com/D221159