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 58eaec2eb326301adba392be383f7eb24071b472..dab97c711020f9dbea57597a7ffb330795c4b22d 100644
--- a/ui/aura/desktop.cc
+++ b/ui/aura/desktop.cc
@@ -429,6 +429,10 @@ 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;
   }
 }