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

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

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ActionItem.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ActionItem.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ActionItem.java
index 76138b906ca5c86fe27123dca6b3e5a314d589c0..08cf25e9bfdd0f0151e27e003a0df1a928c58f6c 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ActionItem.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/ActionItem.java
@@ -13,7 +13,7 @@ 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;
@@ -28,8 +28,9 @@ class ActionItem implements NewTabPageItem {
     }
 
     @Override
-    public int getType() {
-        return NewTabPageItem.VIEW_TYPE_ACTION;
+    @ItemViewType
+    protected int getItemViewType() {
+        return ItemViewType.ACTION;
     }
 
     public int getPosition() {
@@ -83,7 +84,7 @@ class ActionItem implements NewTabPageItem {
     }
 
     @Override
-    public void onBindViewHolder(NewTabPageViewHolder holder) {
+    protected void onBindViewHolder(NewTabPageViewHolder holder) {
         assert holder instanceof ViewHolder;
         ((ViewHolder) holder).onBindViewHolder(this);
     }