Selection API: Selection.extend() should throw if rangeCount is 0.

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, options)|
 // 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 732 matching lines @@
           `You should have focus marker and should not have anchor marker if and only if selection is a caret in "${sampleText}".`);
     }
     this.document_.body.innerHTML = sampleText;
     /** @type {!SampleSelection} */
     const selection = Parser.parse(this.document_.body);
     if (selection.isNone)
       return;
     if (this.loadSelectionInTextArea(selection))
       return;
     this.selection_.collapse(selection.anchorNode, selection.anchorOffset);
-    this.selection_.extend(selection.focusNode, selection.focusOffset);
+    if (this.selection_.rangeCount > 0)
+      this.selection_.extend(selection.focusNode, selection.focusOffset);
   }
 
   /**
    * @private
    * @param {!SampleSelection} selection
    * @return {boolean} Returns true if selection is in TEXTAREA.
    */
   loadSelectionInTextArea(selection) {
     /** @type {Node} */
     const enclosingNode = selection.anchorNode.parentNode;
@@ skipping 159 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;
 })();