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

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ChildNode.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;
+
+/**
+ * A node in the tree that has a parent and can notify it about changes.
+ *
+ * This class mostly serves as a convenience base class for implementations of {@link TreeNode}.
+ */
+public abstract class ChildNode implements TreeNode {
+    private final NodeParent mParent;
+
+    protected ChildNode(NodeParent parent) {
+        mParent = parent;
+    }
+
+    protected void notifyItemRangeChanged(int index, int count) {
+        mParent.onItemRangeChanged(this, index, count);
+    }
+
+    protected void notifyItemRangeInserted(int index, int count) {
+        mParent.onItemRangeInserted(this, index, count);
+    }
+
+    protected void notifyItemRangeRemoved(int index, int count) {
+        mParent.onItemRangeRemoved(this, index, count);
+    }
+
+    protected void notifyItemChanged(int index) {
+        notifyItemRangeChanged(index, 1);
+    }
+
+    protected void notifyItemInserted(int index) {
+        notifyItemRangeInserted(index, 1);
+    }
+
+    protected void notifyItemRemoved(int index) {
+        notifyItemRangeRemoved(index, 1);
+    }
+}