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

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

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NodeParent.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NodeParent.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NodeParent.java
new file mode 100644
index 0000000000000000000000000000000000000000..533e6f4981190f0d8c0b030e28524adfcc66fb57
--- /dev/null
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NodeParent.java
@@ -0,0 +1,41 @@
+// 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;
+
+/**
+ * Interface to allow propagating change events upwards in the tree.
+ */
+public interface NodeParent {
+    /**
+     * Notifies that {@code count} items starting at position {@code index} under the {@code child}
+     * have changed.
+     * @param child The child whose items have changed.
+     * @param index The starting position of the range of changed items, relative to the
+     *         {@code child}.
+     * @param count The number of changed items.
+     * @see android.support.v7.widget.RecyclerView.Adapter#notifyItemRangeChanged(int, int)
+     */
+    void onItemRangeChanged(TreeNode child, int index, int count);
+
+    /**
+     * Notifies that {@code count} items starting at position {@code index} under the {@code child}
+     * have been added.
+     * @param child The child to which items have been added.
+     * @param index The starting position of the range of added items, relative to the child.
+     * @param count The number of added items.
+     * @see android.support.v7.widget.RecyclerView.Adapter#notifyItemRangeInserted(int, int)
+     */
+    void onItemRangeInserted(TreeNode child, int index, int count);
+
+    /**
+     * Notifies that {@code count} items starting at position {@code index} under the {@code child}
+     * have been removed.
+     * @param child The child from which items have been removed.
+     * @param index The starting position of the range of removed items, relative to the child.
+     * @param count The number of removed items.
+     * @see android.support.v7.widget.RecyclerView.Adapter#notifyItemRangeRemoved(int, int)
+     */
+    void onItemRangeRemoved(TreeNode child, int index, int count);
+}