WebUI Polymer 2: Update Polymer.Templatizer usage in settings-idle-load.

Need to override |_forwardHostPropV2| method for Polymer 2. |_forwardParentProp|
and |_forwardParentPath| can be removed after Polymer 2 migration is done.

Bug: 849788
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ic77854582a5184fa41fe5dca02724c9f2cc7dc7d
Reviewed-on: https://chromium-review.googlesource.com/1091653Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565470}

chrome/browser/resources/settings/controls/settings_idle_load.js

@@ -24,6 +24,9 @@ Polymer({
   /** @private {?Element} */
   child_: null,
 
+  /** @private {?Element} */
+  instance_: null,
+
   /** @private {number} */
   idleCallback_: 0,
 
@@ -52,12 +55,12 @@ Polymer({
         this.templatize(this.getContentChildren()[0]);
         assert(this.ctor);
 
-        const instance = this.stamp({});
+        this.instance_ = this.stamp({});
 
         assert(!this.child_);
-        this.child_ = instance.root.firstElementChild;
+        this.child_ = this.instance_.root.firstElementChild;
 
-        this.parentNode.insertBefore(instance.root, this);
+        this.parentNode.insertBefore(this.instance_.root, this);
         resolve(this.child_);
 
         this.fire('lazy-loaded');
@@ -68,6 +71,7 @@ Polymer({
   },
 
   /**
+   * TODO(dpapad): Delete this method once migration to Polymer 2 has finished.
    * @param {string} prop
    * @param {Object} value
    */
@@ -77,11 +81,21 @@ Polymer({
   },
 
   /**
+   * TODO(dpapad): Delete this method once migration to Polymer 2 has finished.
    * @param {string} path
    * @param {Object} value
    */
   _forwardParentPath: function(path, value) {
     if (this.child_)
       this.child_._templateInstance.notifyPath(path, value, true);
-  }
+  },
+
+  /**
+   * @param {string} prop
+   * @param {Object} value
+   */
+  _forwardHostPropV2: function(prop, value) {
+    if (this.instance_)
+      this.instance_.forwardHostProp(prop, value);
+  },
 });