Support undoing offline page deletion

chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageBridge.java

Index: chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageBridge.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageBridge.java b/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageBridge.java
index 5a66045246a828ae40c1df0fe5e2add350ff975d..982e64b7a17c2f8c1ebf89be02cf0ea45b806ef9 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageBridge.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageBridge.java
@@ -64,13 +64,20 @@ public final class OfflinePageBridge {
     }
 
     /**
-     * Interface that provides listeners to be notified of changes to the offline page model.
+     * Base empty implementation observer class that provides listeners to be notified of changes to
+     * the offline page model.
      */
-    public interface OfflinePageModelObserver {
+    public abstract static class OfflinePageModelObserver {
         /**
          * Called when the native side of offline pages is loaded and now in usable state.
          */
-        void offlinePageModelLoaded();
+        public void offlinePageModelLoaded() {}
+
+        /**
+         * Called when the native side of offline pages is changed due to adding, removing or
+         * update an offline page.
+         */
+        public void offlinePageModelChanged() {}
     }
 
     private static int getFreeSpacePercentage() {
@@ -244,6 +251,13 @@ public final class OfflinePageBridge {
     }
 
     @CalledByNative
+    private void offlinePageModelChanged() {
+        for (OfflinePageModelObserver observer : mObservers) {
+            observer.offlinePageModelChanged();
+        }
+    }
+
+    @CalledByNative
     private static void createOfflinePageAndAddToList(List<OfflinePageItem> offlinePagesList,
             String url, long bookmarkId, String offlineUrl, long fileSize, int accessCount) {
         offlinePagesList.add(createOfflinePageItem(url, bookmarkId, offlineUrl, fileSize,