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

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ItemGroup.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 java.util.List;
-
-/**
- * A group of items.
- */
-public interface ItemGroup {
-    /**
-     * @return A list of items contained in this group. The list should not be modified.
-     */
-    List<NewTabPageItem> getItems();
-
-    /**
-     * Defines the actions an object can be notified about when there are changes inside of
-     * an {@link ItemGroup}.
-     */
-    interface Observer {
-        /** Notification about items having been changed in the group. */
-        void onItemRangeChanged(ItemGroup group, int itemPosition, int itemCount);
-
-        /** Notification about items having been added to the group. */
-        void onItemRangeInserted(ItemGroup group, int itemPosition, int itemCount);
-
-        /** Notification about items having been removed from the group. */
-        void onItemRangeRemoved(ItemGroup group, int itemPosition, int itemCount);
-    }
-}