[MD settings] handle being detached on cr-lazy-render and settings-idle-render

ui/webui/resources/cr_elements/cr_lazy_render/cr_lazy_render.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.
 
 /**
  * @fileoverview
  * cr-lazy-render is a simple variant of dom-if designed for lazy rendering
  * of elements that are accessed imperatively.
  * Usage:
  *   <template is="cr-lazy-render" id="menu">
@@ skipping 25 matching lines @@
   /**
    * @return {?Element} The element contained in the template, if it has
    *   already been stamped.
    */
   getIfExists: function() {
     return this.child_;
   },
 
   /** @private */
   render_: function() {
-    if (!this.ctor)
-      this.templatize(this);
+    // Only initialize once.
+    assert(!this.ctor);
+    // Must have a parent and child in order to add child to parent.
+    assert(this.parentNode && !this.child_);
+    this.templatize(this);
     var parentNode = this.parentNode;
-    if (parentNode && !this.child_) {
-      var instance = this.stamp({});
-      this.child_ = instance.root.firstElementChild;
-      parentNode.insertBefore(instance.root, this);
-    }
+    var instance = this.stamp({});
+    this.child_ = instance.root.firstElementChild;
+    parentNode.insertBefore(instance.root, this);
   },
 
   /**
    * @param {string} prop
    * @param {Object} value
    */
   _forwardParentProp: function(prop, value) {
     if (this.child_)
       this.child_._templateInstance[prop] = value;
   },
 
   /**
    * @param {string} path
    * @param {Object} value
    */
   _forwardParentPath: function(path, value) {
     if (this.child_)
       this.child_._templateInstance.notifyPath(path, value, true);
   }
 });