Split the class inheritance trees for nsIDragService and nsIDragSession.
Remember that, before the start of this patch series, the inheritance
diagram was:
nsDragService -> nsBaseDragService -> nsIDragService + nsIDragSession
and the only instance was a singleton. We switched it to:
nsDragService -> nsDragSession -> nsBaseDragService -> nsIDragService + nsBaseDragSession -> nsIDragSession
and maintained the singleton. This allowed us to allow us to move things to
the new classes without breaking behavior. We are done with that,
so we can now change the inheritance to its final form:
nsDragService -> nsBaseDragService -> nsIDragService
nsDragSession -> nsBaseDragSession -> nsIDragSession
Of course, we also need to properly construct and release the
nsIDragSessions (formerly part of the singleton), so that is done here as
well. That happens in nsBaseDrag[Service|Session] for parent process drags
and in nsDrag[Service|Session]Proxy for content. This is all fairly
straightforward, except in the case of gtk, where we need to change
some callback behavior.
Differential Revision: https://phabricator.services.mozilla.com/D211084
e844cdeed5