Unify NewTabPageItem and ItemGroup into a single tree-structured interface.

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ProgressItem.java

 // 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.
 
 package org.chromium.chrome.browser.ntp.cards;
 
 /**
  * Represents a progress indicator for the recycler view. A visibility flag can be set to be used
  * by its associated view holder when it is bound.
  *
  * @see ProgressViewHolder
  */
-class ProgressItem implements NewTabPageItem {
+class ProgressItem extends Leaf {
     private boolean mVisible = false;
 
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_PROGRESS;
+    @ItemViewType
+    protected int getItemViewType() {
+        return ItemViewType.PROGRESS;
     }
 
     public boolean isVisible() {
         return mVisible;
     }
 
     public void setVisible(boolean visible) {
         mVisible = visible;
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         assert holder instanceof ProgressViewHolder;
         ((ProgressViewHolder) holder).onBindViewHolder(this);
     }
 }