Implement workspace scrubbing

ash/wm/workspace_controller.h

Index: ash/wm/workspace_controller.h
diff --git a/ash/wm/workspace_controller.h b/ash/wm/workspace_controller.h
index aa84592596c209b92a01e2e708e399c59bf3d70b..91b417cc205afa6283f8cb7813beb80ac2d81e55 100644
--- a/ash/wm/workspace_controller.h
+++ b/ash/wm/workspace_controller.h
@@ -20,6 +20,7 @@ namespace internal {
 
 class ShelfLayoutManager;
 class WorkspaceControllerTestHelper;
+class WorkspaceCycler;
 class WorkspaceEventHandler;
 class WorkspaceManager;
 
@@ -56,6 +57,10 @@ class ASH_EXPORT WorkspaceController
 
   scoped_ptr<WorkspaceManager> workspace_manager_;
 
+  // Cycles through the WorkspaceManager's workspaces in response to a three
+  // finger vertical scroll.
+  scoped_ptr<WorkspaceCycler> workspace_cycler_;
+
   DISALLOW_COPY_AND_ASSIGN(WorkspaceController);
 };