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

ash/wm/workspace/workspace_manager.h

Index: ash/wm/workspace/workspace_manager.h
diff --git a/ash/wm/workspace/workspace_manager.h b/ash/wm/workspace/workspace_manager.h
index cb827a7e961c0a784e1db56842c796ae8070e06c..c79c399f66806052b70ba1b9a03f5ad72cf74c6d 100644
--- a/ash/wm/workspace/workspace_manager.h
+++ b/ash/wm/workspace/workspace_manager.h
@@ -43,6 +43,7 @@ class DesktopBackgroundFadeController;
 class ShelfLayoutManager;
 class WorkspaceCycler;
 class WorkspaceCyclerAnimator;
+class WorkspaceEventHandler;
 class WorkspaceLayoutManager;
 class WorkspaceManagerTest2;
 class Workspace;
@@ -79,6 +80,8 @@ class ASH_EXPORT WorkspaceManager : public ash::ShellObserver {
   // Returns the container window for the active workspace, never NULL.
   aura::Window* GetActiveWorkspaceWindow();
 
+  WorkspaceEventHandler* GetEventHandlerForWindow(aura::Window* window);
+
   // Returns the parent for |window|. This is invoked from StackingController
   // when a new Window is being added.
   aura::Window* GetParentForNewWindow(aura::Window* window);