Attach panel while dragging to bring it in front of other panels.

ash/wm/window_properties.h

Index: ash/wm/window_properties.h
diff --git a/ash/wm/window_properties.h b/ash/wm/window_properties.h
index 3804f76a0b24fadd5a1d0b69c4c6ea6dc4a119ed..509e3f651546aa7cbe0da1acad939ed699bd9f29 100644
--- a/ash/wm/window_properties.h
+++ b/ash/wm/window_properties.h
@@ -29,6 +29,10 @@ class RootWindowController;
 extern const aura::WindowProperty<internal::AlwaysOnTopController*>* const
     kAlwaysOnTopControllerKey;
 
+// A property key to indicate that an in progress drag should be continued
+// after the window is reparented to another container.
+extern const aura::WindowProperty<bool>* const kContinueDragAfterReparent;
+
 // A property key to indicate that the user is cycling through workspaces.
 // The property should only be set on the root window.
 ASH_EXPORT extern const aura::WindowProperty<bool>* const