[Editing][Codehealth] Rewrite user-select-all-selection.html with assert_selection.

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 497 matching lines @@
    */
   constructor(sampleText) {
     /** @const @type {!HTMLIFame} */
     this.iframe_ = document.createElement('iframe');
     document.body.appendChild(this.iframe_);
     /** @const @type {!HTMLDocument} */
     this.document_ = this.iframe_.contentDocument;
     /** @const @type {!Selection} */
     this.selection_ = this.iframe_.contentWindow.getSelection();
     this.selection_.document = this.document_;
+    this.selection_.document.offsetLeft = this.iframe_.offsetLeft;
+    this.selection_.document.offsetTop = this.iframe_.offsetTop;
 
     this.load(sampleText);
   }
 
   /** @return {!HTMLDocument} */
   get document() { return this.document_; }
 
   /** @return {!Selection} */
   get selection() { return this.selection_; }
 
@@ skipping 122 matching lines @@
   // case.
   if (actualText === expectedText)
     sample.remove();
   assert_equals(actualText, expectedText, description);
 }
 
 // Export symbols
 window.Sample = Sample;
 window.assert_selection = assertSelection;
 })();