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

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

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/AboveTheFoldItem.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/AboveTheFoldItem.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/AboveTheFoldItem.java
index b06b7e1c33569c9074a704a8ff35bec6e04e82f4..6b454f8d70129b1f484b26081783b341223981b3 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/AboveTheFoldItem.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/AboveTheFoldItem.java
@@ -13,14 +13,15 @@ package org.chromium.chrome.browser.ntp.cards;
  * Other elements coming after it and initially off-screen are just added to the RecyclerView after
  * that.
  */
-class AboveTheFoldItem extends SingleItemGroup {
+class AboveTheFoldItem extends Leaf {
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_ABOVE_THE_FOLD;
+    @ItemViewType
+    protected int getItemViewType() {
+        return ItemViewType.ABOVE_THE_FOLD;
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         // Nothing to do.
     }
 }