This patch adds NextSibling() to FrameTreeNode.

content/browser/frame_host/frame_tree_node.h

Index: content/browser/frame_host/frame_tree_node.h
diff --git a/content/browser/frame_host/frame_tree_node.h b/content/browser/frame_host/frame_tree_node.h
index d678c9a53903e24403b6dbbf84c81ab9a125aa8a..0595dd624e0036ebaca3781691641aadf9f3c78d 100644
--- a/content/browser/frame_host/frame_tree_node.h
+++ b/content/browser/frame_host/frame_tree_node.h
@@ -177,6 +177,10 @@ class CONTENT_EXPORT FrameTreeNode {
   // |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;
 
@@ -233,6 +237,8 @@ class CONTENT_EXPORT FrameTreeNode {
  private:
   class OpenerDestroyedObserver;
 
+  FrameTreeNode* GetSibling(int relative_offset) const;
+
   void set_parent(FrameTreeNode* parent) { parent_ = parent; }
 
   // The next available browser-global FrameTreeNode ID.