We need the assertion because we restricted WorkerRunnable::Cancel() must be called in the override Cancel() before WorkerRunnable refactoring. However, we removed this restriction, so we probably no longer need this assertion, although WorkerRunnable::Cancel() still should be called once only.
Bug 1749725 Comment 3 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
We don't need assertion since WorkerRunnable::Cancel() no need to be called in inherited Cancel() implementation.