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

chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.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 android.graphics.Bitmap;
 
-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;
 import org.chromium.chrome.browser.ntp.snippets.ContentSuggestionsCardLayout.ContentSuggestionsCardLayoutEnum;
 
 /**
  * Represents the data for an article card on the NTP.
  */
-public class SnippetArticle implements NewTabPageItem {
+public class SnippetArticle extends Leaf {
     /** The category of this article. */
     public final int mCategory;
 
     /** The identifier for this article within the category - not necessarily unique globally. */
     public final String mIdWithinCategory;
 
     /** The title of this article. */
     public final String mTitle;
 
     /** The canonical publisher name (e.g., New York Times). */
@@ skipping 55 matching lines @@
         SnippetArticle rhs = (SnippetArticle) other;
         return mCategory == rhs.mCategory && mIdWithinCategory.equals(rhs.mIdWithinCategory);
     }
 
     @Override
     public int hashCode() {
         return mCategory ^ mIdWithinCategory.hashCode();
     }
 
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_SNIPPET;
+    @ItemViewType
+    public int getItemViewType() {
+        return ItemViewType.VIEW_TYPE_SNIPPET;
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         assert holder instanceof SnippetArticleViewHolder;
         ((SnippetArticleViewHolder) holder).onBindViewHolder(this);
     }
 
     /**
      * Returns this article's thumbnail as a {@link Bitmap}. Can return {@code null} as it is
      * initially unset.
      */
     public Bitmap getThumbnailBitmap() {
         return mThumbnailBitmap;
@@ skipping 11 matching lines @@
         mImpressionTracked = true;
         return true;
     }
 
     @Override
     public String toString() {
         // For debugging purposes. Displays the first 42 characters of the title.
         return String.format("{%s, %1.42s}", getClass().getSimpleName(), mTitle);
     }
 }