Adding `content_security_policy` to a few sample extensions.

chrome/common/extensions/docs/examples/extensions/news_a11y/feed_iframe.js

+// Copyright (c) 2011 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.
+
+function reportHeight() {
+  var msg = JSON.stringify({type:"size", size:document.body.offsetHeight});
+  parent.postMessage(msg, "*");
+}
+
+function frameLoaded() {
+  var links = document.getElementsByTagName("A");
+  for (i = 0; i < links.length; i++) {
+    var c = links[i].className;
+    if (c != "item_title" && c != "open_box") {
+      links[i].addEventListener("click", showStory);
+    }
+  }
+  window.addEventListener("message", messageHandler);
+}
+
+function showStory(event) {
+  var href = event.currentTarget.href;
+  parent.postMessage(JSON.stringify({type:"show", url:href}), "*");
+  event.preventDefault();
+}
+
+function messageHandler(event) {
+  reportHeight();
+}
+
+document.addEventListener('DOMContentLoaded', frameLoaded);