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

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

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ProgressItem.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ProgressItem.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ProgressItem.java
index 428c97fed34e7b3c119644dd7ca9bf9d0b34775a..788aa7f6f55b0bd031b8a82ef5e9c0766a16542f 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ProgressItem.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ProgressItem.java
@@ -10,12 +10,13 @@ package org.chromium.chrome.browser.ntp.cards;
  *
  * @see ProgressViewHolder
  */
-class ProgressItem implements NewTabPageItem {
+class ProgressItem extends Leaf {
     private boolean mVisible = false;
 
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_PROGRESS;
+    @ItemViewType
+    protected int getItemViewType() {
+        return ItemViewType.PROGRESS;
     }
 
     public boolean isVisible() {
@@ -27,7 +28,7 @@ class ProgressItem implements NewTabPageItem {
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         assert holder instanceof ProgressViewHolder;
         ((ProgressViewHolder) holder).onBindViewHolder(this);
     }