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

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/TreeNode.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 tree interface to allow the New Tab Page RecyclerView to delegate to other components.
+ */
+interface TreeNode {
+    /**
+     * @return The number of items under this subtree.
+     * @see android.support.v7.widget.RecyclerView.Adapter#getItemCount()
+     */
+    int getItemCount();
+
+    /**
+     * @param position The position to query
+     * @return The view type of the item at {@code position} under this subtree.
+     * @see android.support.v7.widget.RecyclerView.Adapter#getItemViewType
+     */
+    @ItemViewType
+    int getItemViewType(int position);
+
+    /**
+     * Display the data at {@code position} under this subtree.
+     * @param holder The view holder that should be updated.
+     * @param position The position of the item under this subtree.
+     * @see android.support.v7.widget.RecyclerView.Adapter#onBindViewHolder
+     */
+    void onBindViewHolder(NewTabPageViewHolder holder, final int position);
+
+    /**
+     * @param position The position to query.
+     * @return The article at {@code position} under this subtree, or null if the item is not
+     * an article.
+     */
+    SnippetArticle getSuggestionAt(int position);
+}