Fix popunders spawned from a subframe.

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 7368c7f4a8611b23f72650560f58bb3dde748569..38c0ed2e58aa0809f6eb9af4224ce2ad59843690 100644
--- a/content/browser/frame_host/frame_tree_node.h
+++ b/content/browser/frame_host/frame_tree_node.h
@@ -124,7 +124,8 @@ class CONTENT_EXPORT FrameTreeNode {
 
   // Assigns the initial opener for this node, and if |opener| is non-null,
   // registers an observer that will clear this node's opener if |opener| is
-  // ever destroyed.
+  // ever destroyed. The value set here is the root of the tree.
+  //
   // It is not possible to change the opener once it was set.
   void SetOriginalOpener(FrameTreeNode* opener);