Linux: Add drag&drop support (issue #1258).

- Requires proper handling of the "XdndProxy" property on the top-level window. This currently works in cefsimple but not cefclient (perhaps due to https://bugzilla.gnome.org/show_bug.cgi?id=653264).

git-svn-id: https://chromiumembedded.googlecode.com/svn/trunk@1759 5089003a-bbd8-11dd-ad1f-f1f9622dbc98

patch/patches/ui_dragdrop_355390.patch

@@ -0,0 +1,14 @@
+Index: os_exchange_data_provider_aurax11.cc
+===================================================================
+--- os_exchange_data_provider_aurax11.cc	(revision 280796)
++++ os_exchange_data_provider_aurax11.cc	(working copy)
+@@ -155,7 +155,8 @@
+     format_map_.Insert(atom_cache_.GetAtom(kMimeTypeMozillaURL), mem);
+ 
+     // Set a string fallback as well.
+-    SetString(spec);
++    if (!HasString())
++      SetString(spec);
+ 
+     // Return early if this drag already contains file contents (this implies
+     // that file contents must be populated before URLs). Nautilus (and possibly