Implement workspace scrubbing

ash/wm/workspace/workspace.h

Index: ash/wm/workspace/workspace.h
diff --git a/ash/wm/workspace/workspace.h b/ash/wm/workspace/workspace.h
index 575a3683ebda79af79e6c9883d96c9689fbff914..1d7dc45ebca1a34d0f0a629bb0e9a4664217985f 100644
--- a/ash/wm/workspace/workspace.h
+++ b/ash/wm/workspace/workspace.h
@@ -55,6 +55,10 @@ class ASH_EXPORT Workspace {
   }
   WorkspaceManager* workspace_manager() { return workspace_manager_; }
 
+  // Returns the active window if the active window is in this workspace.
+  // Otherwise, NULL is returned.
+  aura::Window* GetActiveWindow() const;
+
   // Returns true if the Workspace should be moved to pending. This is true
   // if there are no visible maximized windows.
   bool ShouldMoveToPending() const;