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

chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
index c47c40cd80e8cce10be2eda5398a9423e5d68cfb..de06a3c3608cffc48ac8556c941f21e3d5050ecf 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
@@ -5,14 +5,15 @@ 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;
 
@@ -88,16 +89,24 @@ public class SnippetArticle implements NewTabPageItem {
     }
 
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_SNIPPET;
+    @ItemViewType
+    public int getItemViewType() {
+        return ItemViewType.SNIPPET;
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         assert holder instanceof SnippetArticleViewHolder;
         ((SnippetArticleViewHolder) holder).onBindViewHolder(this);
     }
 
+    @Override
+    public SnippetArticle getSuggestionAt(int position) {
+        if (position != 0) throw new IndexOutOfBoundsException();
+
+        return this;
+    }
+
     /**
      * Returns this article's thumbnail as a {@link Bitmap}. Can return {@code null} as it is
      * initially unset.