[DevTools] Move user agent override to responsive design toolbar.

Source/devtools/front_end/common/DOMExtension.js

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 316 matching lines @@
 {
     return this.ownerDocument.evaluate(query, this, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
 }
 
 Element.prototype.removeChildren = function()
 {
     if (this.firstChild)
         this.textContent = "";
 }
 
+Element.prototype.appendChildren = function(children)
+{
+    for (var i = 0; i < children.length; ++i)
+        this.appendChild(children[i]);
+}
+
+Element.prototype.setChildren = function(children)
+{
+    this.removeChildren();
+    this.appendChildren(children);
+}
+
 Element.prototype.isInsertionCaretInside = function()
 {
     var selection = window.getSelection();
     if (!selection.rangeCount || !selection.isCollapsed)
         return false;
     var selectionRange = selection.getRangeAt(0);
     return selectionRange.startContainer.isSelfOrDescendant(this);
 }
 
 /**
@@ skipping 314 matching lines @@
  */
 function isEnterKey(event) {
     // Check if in IME.
     return event.keyCode !== 229 && event.keyIdentifier === "Enter";
 }
 
 function consumeEvent(e)
 {
     e.consume();
 }