Avoid locking up the web view when starting a drag

We currently don't have an implementation for drag and drop and
we need to explicitly call SystemDragEnded to avoid leaving the
render process in an inconsisten state.

See http://crbug.com/157134

Change-Id: I52220602d4e4832d11ef601589a588c91bb81285
Reviewed-by: Andras Becsi <andras.becsi@digia.com>