[Suggestions] Remove TreeNode.getSuggestionAt() in favor of a visitor.

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

+// Copyright 2017 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;
+import org.chromium.chrome.browser.suggestions.ContentSuggestionsAdditionalAction;
+
+/**
+ * Allows implementing a visitor pattern to iterate over all items under a (sub-)tree.
+ */
+public class NodeVisitor {
+    /**
+     * Visits the above-the-fold item.
+     */
+    public void visitAboveTheFoldItem() {}
+
+    /**
+     * Visits an action item.
+     * @param currentAction The action enum value for the item.
+     */
+    public void visitActionItem(@ContentSuggestionsAdditionalAction int currentAction) {}
+
+    /**
+     * Visits the "all dimissed" item.
+     */
+    public void visitAllDismissedItem() {}
+
+    /**
+     * Visits the footer.
+     */
+    public void visitFooter() {}
+
+    /**
+     * Visits a progress item.
+     */
+    public void visitProgressItem() {}
+
+    /**
+     * Visits a sign-in promo.
+     */
+    public void visitSignInPromo() {}
+
+    /**
+     * Visits a spacing item.
+     */
+    public void visitSpacingItem() {}
+
+    /**
+     * Visits a "no suggestions" item.
+     */
+    public void visitNoSuggestionsItem() {}
+
+    /**
+     * Visits a suggestion.
+     * @param suggestion The {@link SnippetArticle} represented by the item.
+     */
+    public void visitSuggestion(SnippetArticle suggestion) {}
+
+    /**
+     * Visits a header.
+     */
+    public void visitHeader() {}
+
+    /**
+     * Visits the tile grid.
+     */
+    public void visitTileGrid() {}
+}