[Android NTP] Move dismissing items into the TreeNode interface.

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 1a3b7ffa84d402635f3e701a50c10a6bc6b335ff..fe95e48e7e90c380b6cdbf1ddee706011752f1a6 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
@@ -6,6 +6,7 @@ package org.chromium.chrome.browser.ntp.cards;
 
 import android.support.annotation.CallSuper;
 
+import org.chromium.base.Callback;
 import org.chromium.chrome.browser.ntp.snippets.SnippetArticle;
 
 /**
@@ -43,12 +44,17 @@ public abstract class OptionalLeaf extends ChildNode {
     }
 
     @Override
+    public void dismissItem(int position, Callback<String> itemRemovedCallback) {
+        checkIndex(position);
+        dismiss(itemRemovedCallback);
+    }
+
+    @Override
     public int getDismissSiblingPosDelta(int position) {
         checkIndex(position);
         return 0;
     }
 
-
     /** @return Whether the optional item is currently visible. */
     public final boolean isVisible() {
         return mVisible;
@@ -86,9 +92,14 @@ public abstract class OptionalLeaf extends ChildNode {
     @ItemViewType
     protected abstract int getItemViewType();
 
-    protected void checkIndex(int position) {
-        if (position < 0 || position >= getItemCount()) {
-            throw new IndexOutOfBoundsException(position + "/" + getItemCount());
-        }
+    /**
+     * 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
+     * it for accessibility purposes.
+     * @see TreeNode#dismissItem
+     */
+    protected void dismiss(Callback<String> itemRemovedCallback) {
+        assert false;
     }
 }