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

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/Leaf.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 org.chromium.chrome.browser.ntp.snippets.SnippetArticle;
+
+/**
+ * A leaf in the tree, i.e. a single item.
+ */
+public abstract class Leaf implements TreeNode {
+    @Override
+    public int getItemCount() {
+        return 1;
+    }
+
+    @Override
+    @ItemViewType
+    public int getItemViewType(int position) {
+        if (position != 0) throw new IndexOutOfBoundsException();
+        return getItemViewType();
+    }
+
+    @Override
+    public void onBindViewHolder(NewTabPageViewHolder holder, int position) {
+        if (position != 0) throw new IndexOutOfBoundsException();
+        onBindViewHolder(holder);
+    }
+
+    @Override
+    public SnippetArticle getSuggestionAt(int position) {
+        if (position != 0) throw new IndexOutOfBoundsException();
+
+        return null;
+    }
+
+    /**
+     * Display the data for this item.
+     * @param holder The view holder that should be updated.
+     * @see #onBindViewHolder(NewTabPageViewHolder, int)
+     * @see android.support.v7.widget.RecyclerView.Adapter#onBindViewHolder
+     */
+    protected abstract void onBindViewHolder(NewTabPageViewHolder holder);
+
+    /**
+     * @return The view type of this item.
+     * @see android.support.v7.widget.RecyclerView.Adapter#getItemViewType
+     */
+    @ItemViewType
+    protected abstract int getItemViewType();
+}