First shot at implementing drag&drop for Aura

ui/aura/desktop.cc

Index: ui/aura/desktop.cc
diff --git a/ui/aura/desktop.cc b/ui/aura/desktop.cc
index d08a062c1888b750b34f309433c89db179e8f453..a29a63f30c177592e1b6770b978a7daedbeb3d19 100644
--- a/ui/aura/desktop.cc
+++ b/ui/aura/desktop.cc
@@ -14,6 +14,7 @@
 #include "base/string_number_conversions.h"
 #include "base/string_split.h"
 #include "ui/aura/aura_switches.h"
+#include "ui/aura/client/drag_drop_client.h"
 #include "ui/aura/client/stacking_client.h"
 #include "ui/aura/desktop_host.h"
 #include "ui/aura/desktop_observer.h"
@@ -196,6 +197,10 @@ void Desktop::SetStackingClient(StackingClient* stacking_client) {
   stacking_client_.reset(stacking_client);
 }
 
+void Desktop::SetDragDropClient(DragDropClient* drag_drop_client) {
+  drag_drop_client_.reset(drag_drop_client);
+}
+
 void Desktop::ShowDesktop() {
   host_->Show();
 }
@@ -411,6 +416,10 @@ void Desktop::SetCapture(Window* window) {
       mouse_pressed_handler_ = capture_window_;
     if (touch_event_handler_)
       touch_event_handler_ = capture_window_;
+  } else {
+    // When capture is lost, we must reset the event handlers.
+    mouse_pressed_handler_ = NULL;
+    touch_event_handler_ = NULL;
   }
 }