ui/base/models: Add empty() accessor to TreeNode class.

ui/base/models/tree_node_model.h

Index: ui/base/models/tree_node_model.h
diff --git a/ui/base/models/tree_node_model.h b/ui/base/models/tree_node_model.h
index 29443f938f971ef1569f454e5c0686ceb6e4182e..e0f129b5a431b6c64e62d9eadae3601ce545561b 100644
--- a/ui/base/models/tree_node_model.h
+++ b/ui/base/models/tree_node_model.h
@@ -112,6 +112,9 @@ class TreeNode : public TreeModelNode {
   // Returns the number of children.
   int child_count() const { return static_cast<int>(children_->size()); }
 
+  // Returns true if this node has no children.
+  bool empty() const { return children_.empty(); }
+
   // Returns the number of all nodes in the subtree rooted at this node,
   // including this node.
   int GetTotalNodeCount() const {