Support frame tree propagation between renderers in the same browsing instance.

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 76ea08d996eb60776fbac0981c42eded634115a4..6d3982ec1310690aed1367eb5c2df2dcb89068d7 100644
--- a/content/test/data/post_message.html
+++ b/content/test/data/post_message.html
@@ -14,6 +14,13 @@
     w.postMessage(msg, "*");
     return true;
   }
+  
+  // Send a message to a subframe of window named "foo".
+  function postToFooFrame(msg) {
+    var w = window.open("", "foo");
+    w.frames[0][0].postMessage(msg, "*");
+    return true;
+  }
 
   // Listen to incoming messages.
   var receivedMessages = 0;