This patch adds NextSibling() to FrameTreeNode.

content/browser/frame_host/frame_tree_node.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_NODE_H_
 #define CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_NODE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 159 matching lines @@
   RenderFrameHostImpl* current_frame_host() const {
     return render_manager_.current_frame_host();
   }
 
   bool IsDescendantOf(FrameTreeNode* other) const;
 
   // Return the node immediately preceding this node in its parent's
   // |children_|, or nullptr if there is no such node.
   FrameTreeNode* PreviousSibling() const;
 
+  // Return the node immediately after this node in its parent's
+  // |children_|, or nullptr if there is no such node.
+  FrameTreeNode* NextSibling() const;
+
   // Returns true if this node is in a loading state.
   bool IsLoading() const;
 
   // Returns this node's loading progress.
   double loading_progress() const { return loading_progress_; }
 
   NavigationRequest* navigation_request() { return navigation_request_.get(); }
 
   // PlzNavigate
   // Takes ownership of |navigation_request| and makes it the current
@@ skipping 36 matching lines @@
   // Returns the time this frame was last focused.
   base::TimeTicks last_focus_time() const { return last_focus_time_; }
 
   // Called when this node becomes focused.  Updates the node's last focused
   // time and notifies observers.
   void DidFocus();
 
  private:
   class OpenerDestroyedObserver;
 
+  FrameTreeNode* GetSibling(int relative_offset) const;
+
   void set_parent(FrameTreeNode* parent) { parent_ = parent; }
 
   // The next available browser-global FrameTreeNode ID.
   static int next_frame_tree_node_id_;
 
   // The FrameTree that owns us.
   FrameTree* frame_tree_;  // not owned.
 
   // The Navigator object responsible for managing navigations at this node
   // of the frame tree.
@@ skipping 71 matching lines @@
   base::ObserverList<Observer> observers_;
 
   base::TimeTicks last_focus_time_;
 
   DISALLOW_COPY_AND_ASSIGN(FrameTreeNode);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_NODE_H_