This patch adds NextSibling() to FrameTreeNode.

content/browser/frame_host/frame_tree_node.cc

 // 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.
 
 #include "content/browser/frame_host/frame_tree_node.h"
 
 #include <queue>
 #include <utility>
 
 #include "base/macros.h"
@@ skipping 259 matching lines @@
 
   for (FrameTreeNode* node = parent(); node; node = node->parent()) {
     if (node == other)
       return true;
   }
 
   return false;
 }
 
 FrameTreeNode* FrameTreeNode::PreviousSibling() const {
-  if (!parent_)
-    return nullptr;
+  return GetSibling(-1);
+}
 
-  for (size_t i = 0; i < parent_->child_count(); ++i) {
-    if (parent_->child_at(i) == this)
-      return (i == 0) ? nullptr : parent_->child_at(i - 1);
-  }
-
-  NOTREACHED() << "FrameTreeNode not found in its parent's children.";
-  return nullptr;
+FrameTreeNode* FrameTreeNode::NextSibling() const {
+  return GetSibling(1);
 }
 
 bool FrameTreeNode::IsLoading() const {
   RenderFrameHostImpl* current_frame_host =
       render_manager_.current_frame_host();
   RenderFrameHostImpl* pending_frame_host =
       render_manager_.pending_frame_host();
 
   DCHECK(current_frame_host);
 
@@ skipping 187 matching lines @@
 
 void FrameTreeNode::TraceSnapshot() const {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(
       "navigation", "FrameTreeNode",
       TRACE_ID_WITH_SCOPE("FrameTreeNode", frame_tree_node_id_),
       std::unique_ptr<base::trace_event::ConvertableToTraceFormat>(
           new TracedFrameTreeNode(*this)));
 }
 
+FrameTreeNode* FrameTreeNode::GetSibling(int relative_offset) const {
+  if (!parent_)
+    return nullptr;
+
+  for (size_t i = 0; i < parent_->child_count(); ++i) {
+    if (parent_->child_at(i) == this) {
+      if ((relative_offset < 0 && static_cast<size_t>(-relative_offset) > i) ||
+          i + relative_offset >= parent_->child_count()) {
+        return nullptr;
+      }
+      return parent_->child_at(i + relative_offset);
+    }
+  }
+
+  NOTREACHED() << "FrameTreeNode not found in its parent's children.";
+  return nullptr;
+}
+
 }  // namespace content