[Extensions] Expand capabilities to about:blank iframes within a page

chrome/test/data/extensions/content_capabilities/capability_tests.js

 // Copyright 2014 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() {
 
+var getIframe = function() { return document.querySelector('iframe'); };
+
 window.tests = {
   canReadClipboard: function() {
     domAutomationController.send(document.execCommand('paste'));
   },
 
   canWriteClipboard: function() {
     domAutomationController.send(document.execCommand('copy'));
   },
+
+  canReadClipboardInAboutBlankFrame: function() {
+    domAutomationController.send(
+        getIframe().contentDocument.execCommand('paste'));
+  },
+
+  canWriteClipboardInAboutBlankFrame: function() {
+    domAutomationController.send(
+        getIframe().contentDocument.execCommand('copy'));
+  },
 };
 
 }());