SyncFS: Deleting iterator before closing database.

leveldb iterators must be deleted before closing the database or
else a crash will ensue.

BUG=581474

Review URL: https://codereview.chromium.org/1639043002

Cr-Commit-Position: refs/heads/master@{#371712}

chrome/browser/sync_file_system/local/local_file_change_tracker.cc

@@ -599,6 +599,7 @@ SyncStatusCode LocalFileChangeTracker::TrackerDB::GetDirtyEntries(
       LOG(WARNING) << "Failed to deserialize an URL. "
                    << "TrackerDB might be corrupted.";
       db_status_ = SYNC_DATABASE_ERROR_CORRUPTION;
+      iter.reset();  // Must delete before closing the database.
       db_.reset();
       return db_status_;
     }