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

ash/wm/workspace_controller.cc

Index: ash/wm/workspace_controller.cc
diff --git a/ash/wm/workspace_controller.cc b/ash/wm/workspace_controller.cc
index c880663ac8c3c7de9b4c9084f953f65a5a55f842..ba7a581472f827a3232b4ff5b7261db597993475 100644
--- a/ash/wm/workspace_controller.cc
+++ b/ash/wm/workspace_controller.cc
@@ -42,6 +42,11 @@ aura::Window* WorkspaceController::GetActiveWorkspaceWindow() {
   return workspace_manager_->GetActiveWorkspaceWindow();
 }
 
+WorkspaceEventHandler* WorkspaceController::GetEventHandlerForWindow(
+    aura::Window* window) {
+  return workspace_manager_->GetEventHandlerForWindow(window);
+}
+
 aura::Window* WorkspaceController::GetParentForNewWindow(aura::Window* window) {
   return workspace_manager_->GetParentForNewWindow(window);
 }