Fix extension bindings injection for iframes (reland)

chrome/test/data/extensions/api_test/bindings/external_message_listener/background.js

+// Copyright 2016 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.
+
+var messagesReceived = [];
+
+// Have we received the real message from the sender extension?
+var receivedRealSenderMessage = false;
+
+// Has the c++ code in the browser test asked us for the total count of messages
+// we've received?
+var sendCountAfterSenderMessage = false;
+
+function getMessageCountAfterReceivingRealSenderMessage() {
+  if (receivedRealSenderMessage) {
+    window.domAutomationController.send(messagesReceived.length);
+  } else {
+    sendCountAfterSenderMessage = true;
+  }
+}
+
+chrome.runtime.onMessageExternal.addListener(function(msg, sender, respond) {
+  messagesReceived.push({msg:msg, sender:sender});
+  if (msg == 'from_sender') {
+    receivedRealSenderMessage = true;
+    if (sendCountAfterSenderMessage) {
+      window.domAutomationController.send(messagesReceived.length);
+    }
+  }
+});
+
+chrome.test.sendMessage('receiver_ready');