[BackgroundSync] Fix accidental fall-through in getRegistrations callack

There was a missing break in the switch for handling the error state in
BackgroundSyncProvider::GetRegistrationsCallback.

BUG=488451

Review URL: https://codereview.chromium.org/1130313003

Cr-Commit-Position: refs/heads/master@{#330088}

content/child/background_sync/background_sync_provider.cc

@@ -213,6 +213,7 @@ void BackgroundSyncProvider::GetRegistrationsCallback(
                             registrations[i]).release();
       }
       callbacks->onSuccess(results);
+      break;
     case BACKGROUND_SYNC_ERROR_NOT_FOUND:
       // This error should never be returned from
       // BackgroundSyncManager::GetRegistrations