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

chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SectionHeader.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.snippets;
 
-import org.chromium.chrome.browser.ntp.cards.NewTabPageItem;
+import org.chromium.chrome.browser.ntp.cards.ItemViewType;
+import org.chromium.chrome.browser.ntp.cards.Leaf;
 import org.chromium.chrome.browser.ntp.cards.NewTabPageViewHolder;
 
 /**
  * Represents the data for a header of a group of snippets
  */
-public class SectionHeader implements NewTabPageItem {
+public class SectionHeader extends Leaf {
     /** Whether the header should be shown. */
     private final boolean mVisible;
 
     /** The header text to be shown. */
     private final String mHeaderText;
 
     public SectionHeader(String headerText) {
         // TODO(mvanouwerkerk): Configure mVisible in the constructor when we have a global status
         // section without a visible header.
         mVisible = true;
 
         this.mHeaderText = headerText;
     }
 
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_HEADER;
+    @ItemViewType
+    public int getItemViewType() {
+        return ItemViewType.VIEW_TYPE_HEADER;
     }
 
     public boolean isVisible() {
         return mVisible;
     }
 
     public String getHeaderText() {
         return mHeaderText;
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         assert holder instanceof SectionHeaderViewHolder;
         ((SectionHeaderViewHolder) holder).onBindViewHolder(this);
     }
 }