There has been known corruption of files in the isolated tree. For general safety concerns, we should always verify the items in the isolated cache that were mapped in the tree, after a task completed.

There's two options:
- This is done after the task is completed, to not increase the task overall latency
- This is done during the task window as overhead, so if an input is modified, we can mark the task as failed.

There isn't that much value in the second so we'll aim for the first. This requires a change in task_runner, and potentially a fallback in bot_main too. I'd be nice to be able to register this overhead, but this probably requires issue 813412. Not marking as a blocker because correctness is more important short term.