Add #add(position, index) to ListModelBase.

Change-Id: I81198806879df502bc3eb1b7522330c3c4a82805
Reviewed-on: https://chromium-review.googlesource.com/c/1491553
Commit-Queue: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636066}

ui/android/java/src/org/chromium/ui/modelutil/ListModelBase.java

@@ -54,6 +54,17 @@ public class ListModelBase<T, P> extends ListObservableImpl<P> implements Simple
         notifyItemInserted(mItems.size() - 1);
     }
 
+    /**
+     * Inserts a given {@code item} at position {@code position} of the held {@link List}.
+     * Notifies observers about the inserted item.
+     * @param position The position of the item to be inserted.
+     * @param item The item to be inserted.
+     */
+    public void add(int position, T item) {
+        mItems.add(position, item);
+        notifyItemInserted(position);
+    }
+
     /**
      * Appends all given {@code items} to the last position of the held {@link List}.
      * Notifies observers about the inserted items.