bug#19885: 24.4; "XdndDrop" event does not reach `x-dnd-handle-drag-n-drop-event' when dragging from Chromium

[Oleh Krehel]

Here are my system parameters:

In GNU Emacs 24.4.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.8.6)
 of 2015-02-03 on firefly
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:     Ubuntu 14.04.1 LTS

When dragging an image link from e.g. Firefox, everything works fine,
"XdndDrop" event is received.

However, when dragging the same link from Chromium, this event is never
received. Other events, like "XdndEnter", "XdndPosition" and "XdndLeave"
are received from both browsers.

I'm not sure if Emacs is the cause, it could be Chromium's fault for not
sending the event. Maybe someone with more knowledge could look at this.

Slightly related, I'd also like to know why sometimes the DND
transaction gets stuck while I'm debugging something in Emacs. Then, I'm
unable to start a new DND from Firefox unless I restart Firefox. Could
something be done on Emacs side to prevent this?