Only show Last N Pages in the UI when the corresponding tab is visible.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageUtils.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageUtils.java b/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageUtils.java
index dc67d57487982bb5856f7b4c23bea943bf651931..4be92b3e1f54925b7d4151d9148c88ccb1a945fa 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageUtils.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageUtils.java
@@ -31,7 +31,9 @@ import org.chromium.chrome.browser.snackbar.Snackbar;
 import org.chromium.chrome.browser.snackbar.SnackbarManager;
 import org.chromium.chrome.browser.snackbar.SnackbarManager.SnackbarController;
 import org.chromium.chrome.browser.tab.Tab;
+import org.chromium.chrome.browser.tabmodel.TabModel;
 import org.chromium.chrome.browser.tabmodel.TabModelSelector;
+import org.chromium.chrome.browser.tabmodel.TabModelSelectorTabModelObserver;
 import org.chromium.components.bookmarks.BookmarkId;
 import org.chromium.components.offlinepages.SavePageResult;
 import org.chromium.content_public.browser.LoadUrlParams;
@@ -45,8 +47,11 @@ import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.nio.channels.FileChannel;
+import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
+import java.util.WeakHashMap;
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -703,6 +708,73 @@ public class OfflinePageUtils {
         return ConnectionType.CONNECTION_UNKNOWN;
     }
 
+    /**
+     * Tracks tab creation and closure for the Recent Tabs feature.  UI needs to stop showing
+     * recent offline pages as soon as the tab is closed.  The TabModel is used to get profile
+     * information because Tab's profile is tied to the native WebContents, which may not exist at
+     * tab adding or tab closing time.
+     */
+    private static class RecentTabTracker extends TabModelSelectorTabModelObserver {
+        private TabModelSelector mTabModelSelector;
+
+        public RecentTabTracker(TabModelSelector selector) {
+            super(selector);
+            mTabModelSelector = selector;
+        }
+
+        @Override
+        public void didAddTab(Tab tab, TabModel.TabLaunchType type) {
+            Profile profile = mTabModelSelector.getModel(tab.isIncognito()).getProfile();
+            OfflinePageBridge bridge = OfflinePageBridge.getForProfile(profile);
+            if (bridge == null) return;
+            bridge.registerRecentTab(tab.getId());
+        }
+
+        @Override
+        public void didCloseTab(int tabId, boolean incognito) {
+            Profile profile = mTabModelSelector.getModel(incognito).getProfile();
+            OfflinePageBridge bridge = OfflinePageBridge.getForProfile(profile);
+            if (bridge == null) return;
+
+            // First, unregister the tab with the UI.
+            bridge.unregisterRecentTab(tabId);
+
+            // Then, delete any "Last N" offline pages as well.  This is an optimization because
+            // the UI will no longer show the page, and the page would also be cleaned up by GC
+            // given enough time.
+            ClientId clientId =
+                    new ClientId(OfflinePageBridge.LAST_N_NAMESPACE, Integer.toString(tabId));
+            List<ClientId> clientIds = new ArrayList<>();
+            clientIds.add(clientId);
+
+            bridge.deletePagesByClientId(clientIds, new Callback<Integer>() {
+                @Override
+                public void onResult(Integer result) {
+                    // Result is ignored.
+                }
+            });
+        }
+    }
+
+    /**
+     * Tracks the observers of ChromeActivity's TabModelSelectors. This is weak so the activity can
+     * be garbage collected without worrying about this map.  The RecentTabTracker is held here so
+     * that it can be destroyed when the ChromeActivity gets a new TabModelSelector.
+     */
+    private static WeakHashMap<ChromeActivity, RecentTabTracker> sTabModelObservers =
+            new WeakHashMap<>();
+
+    /**
+     * Starts tracking the tab models in the given selector for tab addition and closure,
+     * destroying obsolete observers as necessary.
+     */
+    public static void observeTabModelSelector(
+            ChromeActivity activity, TabModelSelector tabModelSelector) {
+        RecentTabTracker previousObserver =
+                sTabModelObservers.put(activity, new RecentTabTracker(tabModelSelector));
+        if (previousObserver != null) previousObserver.destroy();
+    }
+
     @VisibleForTesting
     static void setInstanceForTesting(OfflinePageUtils instance) {
         sInstance = instance;