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 a8a9eedb88c3a255ca36b7418eb74607a396239d..e65e142e24bbc8a4716618951ea16abb8a1e8a99 100644
--- a/content/browser/frame_host/frame_tree_node.h
+++ b/content/browser/frame_host/frame_tree_node.h
@@ -206,6 +206,10 @@ class CONTENT_EXPORT FrameTreeNode {
   // |children_|, or nullptr if there is no such node.
   FrameTreeNode* PreviousSibling() const;
 
+  // Return the node immediately following 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;
 
@@ -276,6 +280,8 @@ class CONTENT_EXPORT FrameTreeNode {
 
   void TraceSnapshot() const;
 
+  FrameTreeNode* GetSibling(int relative_offset) const;
+
   // The next available browser-global FrameTreeNode ID.
   static int next_frame_tree_node_id_;