This patch adds NextSibling() to FrameTreeNode.

content/browser/frame_host/frame_tree_node.cc

Index: content/browser/frame_host/frame_tree_node.cc
diff --git a/content/browser/frame_host/frame_tree_node.cc b/content/browser/frame_host/frame_tree_node.cc
index 73ef0c3d9d084f6d63db8fff506099ec1fc61bac..cf16da9f09492772f1b4c9fec969f24a09e9c318 100644
--- a/content/browser/frame_host/frame_tree_node.cc
+++ b/content/browser/frame_host/frame_tree_node.cc
@@ -277,16 +277,11 @@ bool FrameTreeNode::IsDescendantOf(FrameTreeNode* other) const {
 }
 
 FrameTreeNode* FrameTreeNode::PreviousSibling() const {
-  if (!parent_)
-    return nullptr;
-
-  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);
-  }
+  return GetSibling(-1);
+}
 
-  NOTREACHED() << "FrameTreeNode not found in its parent's children.";
-  return nullptr;
+FrameTreeNode* FrameTreeNode::NextSibling() const {
+  return GetSibling(1);
 }
 
 bool FrameTreeNode::IsLoading() const {
@@ -494,4 +489,23 @@ void FrameTreeNode::TraceSnapshot() const {
           new TracedFrameTreeNode(*this)));
 }
 
+FrameTreeNode* FrameTreeNode::GetSibling(int relative_offset) const {
+  if (!parent_ || !parent_->child_count())
+    return nullptr;
+
+  for (size_t i = 0; i < parent_->child_count(); ++i) {
+    if (parent_->child_at(i) == this) {
+      size_t offset = i + relative_offset;

[Charlie Reis, 2016/05/13 23:18:38]

You're still doing the rollover (or rather, rollunder).  Simple cases like
PreviousSibling of the first frame will hit it.  I understand that it's well
defined, but it just seems like we should avoid it.

Nick suggests CheckedNumerics, but I think we can just rephrase by moving the
negative relative_offset check before this line.  Or we could move the whole
bounds check before and do the i+relative_offset twice (skipping the declaration
of offset), since additions are cheap.

[paulmeyer, 2016/05/16 15:12:21]

Done.

+      if (offset >= parent_->child_count() ||
+          (relative_offset < 0 && static_cast<size_t>(-relative_offset) > i)) {
+        return nullptr;
+      }
+      return parent_->child_at(offset);
+    }
+  }
+
+  NOTREACHED() << "FrameTreeNode not found in its parent's children.";
+  return nullptr;
+}
+
 }  // namespace content