[Android NTP] Move more of the dismissal logic into the tree.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/OptionalLeaf.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/OptionalLeaf.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/OptionalLeaf.java
index fe95e48e7e90c380b6cdbf1ddee706011752f1a6..0cad55604f3771dbf850cc1f7448669a3f13136b 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/OptionalLeaf.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/OptionalLeaf.java
@@ -9,6 +9,9 @@ import android.support.annotation.CallSuper;
 import org.chromium.base.Callback;
 import org.chromium.chrome.browser.ntp.snippets.SnippetArticle;
 
+import java.util.Collections;
+import java.util.Set;
+
 /**
  * An optional leaf (i.e. single item) in the tree. Depending on its internal state (see
  * {@link #isVisible()}), the item will be present or absent from the tree, by manipulating the
@@ -44,15 +47,15 @@ public abstract class OptionalLeaf extends ChildNode {
     }
 
     @Override
-    public void dismissItem(int position, Callback<String> itemRemovedCallback) {
+    public Set<Integer> getItemDismissalGroup(int position) {
         checkIndex(position);
-        dismiss(itemRemovedCallback);
+        return canBeDismissed() ? Collections.singleton(0) : Collections.<Integer>emptySet();
     }
 
     @Override
-    public int getDismissSiblingPosDelta(int position) {
+    public void dismissItem(int position, Callback<String> itemRemovedCallback) {
         checkIndex(position);
-        return 0;
+        dismiss(itemRemovedCallback);
     }
 
     /** @return Whether the optional item is currently visible. */
@@ -93,6 +96,13 @@ public abstract class OptionalLeaf extends ChildNode {
     protected abstract int getItemViewType();
 
     /**
+     * @return Whether the item can be dismissed.
+     */
+    protected boolean canBeDismissed() {
+        return false;
+    }
+
+    /**
      * Dismiss this item. The default implementation asserts, as by default items can't be
      * dismissed.
      * @param itemRemovedCallback Should be called with the title of the dismissed item, to announce