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

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SpacingItem.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;
 
 import android.content.Context;
 import android.view.View;
 import android.view.ViewGroup;
 
 /**
  * Placeholder item to let the snippets flow to the top of the scroll list even when it does not
  * contain enough of them. It is displayed as a dummy item with variable height that just occupies
  * the remaining space between the last item in the RecyclerView and the bottom of the screen.
  */
-public class SpacingItem extends SingleItemGroup {
+public class SpacingItem extends Leaf {
     private static class SpacingView extends View {
         public SpacingView(Context context) {
             super(context);
         }
 
         @Override
         protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
             setMeasuredDimension(
                     0, ((NewTabPageRecyclerView) getParent()).calculateBottomSpacing());
         }
     }
 
     /** Creates the View object for displaying the variable spacing. */
     public static View createView(ViewGroup parent) {
         return new SpacingView(parent.getContext());
     }
 
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_SPACING;
+    @ItemViewType
+    protected int getItemViewType() {
+        return ItemViewType.SPACING;
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         // Nothing to do.
     }
 }