Makes WorkspaceLayoutManager use ash/wm/common types

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 92045828335f40bbfd9c973e5ecf87cbf5588cde..289d4403d92392858a8f160d2a1791e24ecfb433 100644
--- a/ash/wm/workspace_controller.h
+++ b/ash/wm/workspace_controller.h
@@ -57,6 +57,8 @@ class ASH_EXPORT WorkspaceController {
 
   ShelfLayoutManager* shelf_;
   std::unique_ptr<WorkspaceEventHandler> event_handler_;
+
+  // Owned by |viewport_|.
   WorkspaceLayoutManager* layout_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(WorkspaceController);