[HTML Imports] Make HTMLLinkElement.import a Document.

LayoutTests/http/tests/resources/testharnessreport.js

 /*
  * THIS FILE INTENTIONALLY LEFT BLANK
  *
  * More specifically, this file is intended for vendors to implement
  * code needed to integrate testharness.js tests with their own test systems.
  *
  * Typically such integration will attach callbacks when each test is
  * has run, using add_result_callback(callback(test)), or when the whole test file has
  * completed, using add_completion_callback(callback(tests, harness_status)).
  *
@@ skipping 59 matching lines @@
 
         // Set results element's innerHTML to the results string
         results.innerHTML = resultStr;
 
         // Add results element to document
         document.body.appendChild(results);
 
         if (self.testRunner)
                 testRunner.notifyDone();
 });