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

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageRecyclerView.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.animation.Animator;
 import android.animation.AnimatorListenerAdapter;
 import android.animation.AnimatorSet;
 import android.animation.ObjectAnimator;
@@ skipping 156 matching lines @@
     /**
      * Updates the space added at the end of the list to make sure the above/below the fold
      * distinction can be preserved.
      */
     public void refreshBottomSpacing() {
         ViewHolder bottomSpacingViewHolder = findBottomSpacer();
 
         // It might not be in the layout yet if it's not visible or ready to be displayed.
         if (bottomSpacingViewHolder == null) return;
 
-        assert bottomSpacingViewHolder.getItemViewType() == NewTabPageItem.VIEW_TYPE_SPACING;
+        assert bottomSpacingViewHolder.getItemViewType() == ItemViewType.SPACING;
         bottomSpacingViewHolder.itemView.requestLayout();
     }
 
     /**
      * Calculates the height of the bottom spacing item, such that there is always enough content
      * below the fold to push the header up to to the top of the screen.
      */
     int calculateBottomSpacing() {
         int aboveTheFoldPosition = getNewTabPageAdapter().getAboveTheFoldPosition();
         int firstVisiblePos = mLayoutManager.findFirstVisibleItemPosition();
@@ skipping 329 matching lines @@
     public void updateViewStateForDismiss(float dX, ViewHolder viewHolder) {
         if (!((NewTabPageViewHolder) viewHolder).isDismissable()) return;
 
         viewHolder.itemView.setTranslationX(dX);
 
         float input = Math.abs(dX) / viewHolder.itemView.getMeasuredWidth();
         float alpha = 1 - DISMISS_INTERPOLATOR.getInterpolation(input);
         viewHolder.itemView.setAlpha(alpha);
     }
 }