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

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ActionItem.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 android.view.View;
 
 import org.chromium.chrome.R;
 import org.chromium.chrome.browser.ntp.NewTabPageView.NewTabPageManager;
 import org.chromium.chrome.browser.ntp.UiConfig;
 
 /**
  * Item that allows the user to perform an action on the NTP.
  */
-class ActionItem implements NewTabPageItem {
+class ActionItem extends Leaf {
     private static final String TAG = "NtpCards";
 
     private final SuggestionsCategoryInfo mCategoryInfo;
 
     // The position (index) of this item within its section, for logging purposes.
     private int mPosition;
     private boolean mImpressionTracked = false;
     private boolean mDismissable;
 
     public ActionItem(SuggestionsCategoryInfo categoryInfo) {
         mCategoryInfo = categoryInfo;
     }
 
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_ACTION;
+    @ItemViewType
+    protected int getItemViewType() {
+        return ItemViewType.ACTION;
     }
 
     public int getPosition() {
         return mPosition;
     }
 
     public void setPosition(int position) {
         mPosition = position;
     }
 
@@ skipping 33 matching lines @@
         public boolean isDismissable() {
             return mActionListItem.mDismissable;
         }
 
         public void onBindViewHolder(ActionItem item) {
             mActionListItem = item;
         }
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         assert holder instanceof ViewHolder;
         ((ViewHolder) holder).onBindViewHolder(this);
     }
 
     /** Set whether this item can be dismissed.*/
     public void setDismissable(boolean dismissable) {
         this.mDismissable = dismissable;
     }
 }