Disallow toggling of shelf auto-hide with touch when there are no windows open

ash/shelf/shelf_layout_manager.h

Index: ash/shelf/shelf_layout_manager.h
diff --git a/ash/shelf/shelf_layout_manager.h b/ash/shelf/shelf_layout_manager.h
index 11dff933175288c98f59be611058d962683a74c3..1f56cb73796d2fe9c2922d7cd3563b979fb5350b 100644
--- a/ash/shelf/shelf_layout_manager.h
+++ b/ash/shelf/shelf_layout_manager.h
@@ -261,6 +261,9 @@ class ASH_EXPORT ShelfLayoutManager
   // two displays.
   gfx::Rect GetAutoHideShowShelfRegionInScreen() const;
 
+  // Returns true if at least one window is visible.
+  bool HasVisibleWindow() const;
+
   // Returns the AutoHideState. This value is determined from the shelf and
   // tray.
   ShelfAutoHideState CalculateAutoHideState(