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

ash/root_window_controller.h

Index: ash/root_window_controller.h
diff --git a/ash/root_window_controller.h b/ash/root_window_controller.h
index d10c6b179bc8575bc0361c05fa71c5e2ff72faa4..4b7dc057f1adc68515db2bdcfcb567397d52bc1b 100644
--- a/ash/root_window_controller.h
+++ b/ash/root_window_controller.h
@@ -80,6 +80,10 @@ class ASH_EXPORT RootWindowController {
 
   ScreenDimmer* screen_dimmer() { return screen_dimmer_.get(); }
 
+  ToplevelWindowEventHandler* panel_container_handler() {
+    return panel_container_handler_.get();
+  }
+
   // Access the shelf associated with this root window controller,
   // NULL if no such shelf exists.
   ShelfWidget* shelf() { return shelf_.get(); }