[Extensions] Don't allow extensions to inject scripts into extension pages

chrome/test/data/extensions/api_test/content_scripts/background_page_iframe/background.js

Index: chrome/test/data/extensions/api_test/content_scripts/background_page_iframe/background.js
diff --git a/chrome/test/data/extensions/api_test/content_scripts/background_page_iframe/background.js b/chrome/test/data/extensions/api_test/content_scripts/background_page_iframe/background.js
new file mode 100644
index 0000000000000000000000000000000000000000..8da0bc66325d6a56011e02db6c9d9ab20a37cfc2
--- /dev/null
+++ b/chrome/test/data/extensions/api_test/content_scripts/background_page_iframe/background.js
@@ -0,0 +1,11 @@
+// Copyright 2015 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.
+
+chrome.test.getConfig(function(config) {
+  var iframe = document.createElement('iframe');
+  iframe.src =
+      'http://a.com:' + config.testServer.port + '/extensions/test_file.html';
+  iframe.onload = function() { chrome.test.sendMessage('iframe loaded'); }
+  document.body.appendChild(iframe);
+});