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 5409f415b3f462626a63ba76b9d46fc9b3914c17..0358b0a29f2f56ab1db7189f9fb6073ccf596a6a 100644
--- a/content/browser/frame_host/frame_tree_node.h
+++ b/content/browser/frame_host/frame_tree_node.h
@@ -123,7 +123,8 @@ 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);