Handle frame openers in the same FrameTree when navigating subframes.

content/test/data/post_message.html

Index: content/test/data/post_message.html
diff --git a/content/test/data/post_message.html b/content/test/data/post_message.html
index b56764d1c3c2b55a697d46278b74b6d82ec150e9..f20ca5675367b83d24b6c446e47002ce7f00e804 100644
--- a/content/test/data/post_message.html
+++ b/content/test/data/post_message.html
@@ -14,6 +14,12 @@
     return true;
   }
 
+  // Send a message to our parent's opener.
+  function postToOpenerOfParent(msg, origin) {
+    parent.opener.postMessage(msg, origin);
+    return true;
+  }
+
   // Send a message to a window named "foo".
   function postToFoo(msg) {
     var w = window.open("", "foo");