I think it depends on whether the task was cancelled before or after the worker claimed it. If it was cancelled before the worker claimed it, I think we can count it as not having run.
Bug 1563307 Comment 7 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
~~I think it depends on whether the task was cancelled before or after the worker claimed it. If it was cancelled before the worker claimed it, I think we can count it as not having run.~~ Sorry, this doesn't make sense. A worker wouldn't claim a cancelled task! Ignore me!