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 "base/command_line.h"
 #include "base/profiler/scoped_tracker.h"
@@ skipping 199 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 139 matching lines @@
 
   render_manager_.Stop();
   return true;
 }
 
 void FrameTreeNode::DidFocus() {
   last_focus_time_ = base::TimeTicks::Now();
   FOR_EACH_OBSERVER(Observer, observers_, OnFrameTreeNodeFocused(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) {
+      size_t offset = i + relative_offset;
+      return offset >= parent_->child_count() ? nullptr

[Charlie Reis, 2015/12/09 22:29:15]

Don't we need to check for less than 0 as well?  Seems like we'd get rollover to
MAX_INT for offset, which isn't good.

[paulmeyer, 2015/12/09 22:38:43]

I wasn't checking for less than zero specifically because |offset| is unsigned.
We could get rollover to MAX_INT, but why is that not good?

I actually asked another engineer about this when I coded it because it did seem
a bit weird to me, but he said it was fine and I couldn't think of a good reason
why it wouldn't be fine either.

[Charlie Reis, 2015/12/09 23:49:48]

That would preclude any analyses which check for rollover, plus it's subtle. 
(It's also wrong in extreme cases, if you have enough children and a possibly
large negative offset.)

It's better to just be explicit about it and add the < 0 check.

[paulmeyer, 2016/05/13 13:44:15]

Okay, done.

+          : parent_->child_at(offset);
+    }
+  }
+
+  NOTREACHED() << "FrameTreeNode not found in its parent's children.";
+  return nullptr;
+}
+
 }  // namespace content