Fix the issue while clicking on offline page donwload notification

When OfflinePageDownloadBridge is created, it will load all the
offline page items.
However, openItem() can get called before all page items are loaded.
This causes the native side unable to find the downloaded item.
As a result, nothing happens when user clicks offline page download
success notification.
This CL waits for all the page items to get loaded before calling
openItem().

BUG=640467

Review-Url: https://codereview.chromium.org/2272883004
Cr-Commit-Position: refs/heads/master@{#414317}

chrome/android/java/src/org/chromium/chrome/browser/download/DownloadNotificationService.java

@@ -506,6 +506,7 @@ public class DownloadNotificationService extends Service {
                                 ? DownloadSharedPreferenceEntry.ITEM_TYPE_OFFLINE_PAGE
                                 : DownloadSharedPreferenceEntry.ITEM_TYPE_DOWNLOAD);
                 DownloadServiceDelegate downloadServiceDelegate = getServiceDelegate(itemType);
+                boolean destroyImmediately = true;
                 switch (intent.getAction()) {
                     case ACTION_DOWNLOAD_CANCEL:
                         // TODO(qinmin): Alternatively, we can delete the downloaded content on
@@ -531,15 +532,27 @@ public class DownloadNotificationService extends Service {
                         resumeAllPendingDownloads();
                         break;
                     case ACTION_DOWNLOAD_OPEN:
-                        assert entry == null;
-                        String guid = IntentUtils.safeGetStringExtra(intent, EXTRA_DOWNLOAD_GUID);
-                        downloadServiceDelegate.openItem(guid);
+                        final OfflinePageDownloadBridge bridge =
+                                (OfflinePageDownloadBridge) downloadServiceDelegate;
+                        destroyImmediately = false;
+                        bridge.addObserver(
+                                new OfflinePageDownloadBridge.Observer() {
+                                    @Override
+                                    public void onItemsLoaded() {
+                                        String guid = IntentUtils.safeGetStringExtra(
+                                                intent, EXTRA_DOWNLOAD_GUID);
+                                        bridge.openItem(guid);
+                                        bridge.destroyServiceDelegate();
+                                    }
+                                });
                         break;
                     default:
                         Log.e(TAG, "Unrecognized intent action.", intent);
                         break;
                 }
-                downloadServiceDelegate.destroyServiceDelegate();
+                if (destroyImmediately) {
+                    downloadServiceDelegate.destroyServiceDelegate();
+                }
             }
         };
         try {