Make assert_selection() to support no BODY element case

third_party/WebKit/LayoutTests/editing/assert_selection.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.
 
 'use strict';
 
 // This file provides |assert_selection(sample, tester, expectedText)| assertion
 // to W3C test harness to write editing test cases easier.
 //
 // |sample| is an HTML fragment text which is inserted as |innerHTML|. It should
@@ skipping 448 matching lines @@
     }
     this.serializeChildren(element);
     this.emit(`</${tagName}>`);
   }
 
   /**
    * @public
    * @param {!HTMLDocument} document
    */
   serialize(document) {
-    this.serializeChildren(document.body);
+    if (document.body)
+        this.serializeChildren(document.body);
+    else
+        this.serializeInternal(document.documentElement);
     return this.strings_.join('');
   }
 
   /**
    * @private
    * @param {!HTMLElement} element
    */
   serializeChildren(element) {
     /** @type {!Array<!Node>} */
     const childNodes = Array.from(element.childNodes);
@@ skipping 197 matching lines @@
   throw new Error(`${description}\n` +
     `\t expected ${expectedText},\n` +
     `\t but got  ${actualText},\n` +
     `\t sameupto ${commonPrefixOf(expectedText, actualText)}`);
 }
 
 // Export symbols
 window.Sample = Sample;
 window.assert_selection = assertSelection;
 })();