Fix popunders spawned from a subframe.

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/lazy_instance.h"
@@ skipping 209 matching lines @@
 
   if (opener_) {
     if (!opener_observer_)
       opener_observer_ = base::MakeUnique<OpenerDestroyedObserver>(this, false);
     opener_->AddObserver(opener_observer_.get());
   }
 }
 
 void FrameTreeNode::SetOriginalOpener(FrameTreeNode* opener) {
   DCHECK(!original_opener_ || !opener);
+  DCHECK(opener == nullptr || !opener->parent());
 
   original_opener_ = opener;
 
   if (original_opener_) {
     DCHECK(!original_opener_observer_);
     original_opener_observer_ =
         base::MakeUnique<OpenerDestroyedObserver>(this, true);
     original_opener_->AddObserver(original_opener_observer_.get());
   }
 }
@@ skipping 316 matching lines @@
       }
       return parent_->child_at(i + relative_offset);
     }
   }
 
   NOTREACHED() << "FrameTreeNode not found in its parent's children.";
   return nullptr;
 }
 
 }  // namespace content