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

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

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/Leaf.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/Leaf.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/Leaf.java
index 0d984143df6e66c10ddbc8f276e3be8e078cad56..1df0a871d41b1a9155122001a70d141819e42fe0 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/Leaf.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/Leaf.java
@@ -7,6 +7,9 @@ package org.chromium.chrome.browser.ntp.cards;
 import org.chromium.base.Callback;
 import org.chromium.chrome.browser.ntp.snippets.SnippetArticle;
 
+import java.util.Collections;
+import java.util.Set;
+
 /**
  * A permanent leaf in the tree, i.e. a single item.
  * If the leaf is not to be a permanent member of the tree, see {@link OptionalLeaf} for an
@@ -39,13 +42,13 @@ public abstract class Leaf extends ChildNode {
     }
 
     @Override
-    public void dismissItem(int position, Callback<String> itemRemovedCallback) {
-        assert false;
+    public Set<Integer> getItemDismissalGroup(int position) {
+        return Collections.emptySet();
     }
 
     @Override
-    public int getDismissSiblingPosDelta(int position) {
-        return 0;
+    public void dismissItem(int position, Callback<String> itemRemovedCallback) {
+        assert false;
     }
 
     /**