Fix popunders spawned from a subframe.

BUG=705316
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2773403002
Cr-Commit-Position: refs/heads/master@{#459835}
diff --git a/content/browser/frame_host/frame_tree_node.cc b/content/browser/frame_host/frame_tree_node.cc
index 288bcec..cb345e3c 100644
--- a/content/browser/frame_host/frame_tree_node.cc
+++ b/content/browser/frame_host/frame_tree_node.cc
@@ -227,6 +227,7 @@
 
 void FrameTreeNode::SetOriginalOpener(FrameTreeNode* opener) {
   DCHECK(!original_opener_ || !opener);
+  DCHECK(opener == nullptr || !opener->parent());
 
   original_opener_ = opener;