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

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ProgressItem.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;
 
 /**
  * Represents a progress indicator for the recycler view. A visibility flag can be set to be used
  * by its associated view holder when it is bound.
  *
  * @see ProgressViewHolder
  */
 class ProgressItem extends OptionalLeaf {
 
     @Override
     @ItemViewType
     protected int getItemViewType() {
         return ItemViewType.PROGRESS;
     }
 
     @Override
     protected void onBindViewHolder(NewTabPageViewHolder holder) {
         assert holder instanceof ProgressViewHolder;
         ((ProgressViewHolder) holder).onBindViewHolder(this);
     }
+
+    @Override
+    protected void visitOptionalItem(NodeVisitor visitor) {
+        visitor.visitProgressItem();
+    }
 }