Fix DCHECK from slow_ending_task_closure

The closure needs to be cancelled earlier to avoid hitting a DCHECK.

Bug: 956452
Change-Id: Iba2b9b7cbde0f666278f0e26783c3ab9c577f528
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583741
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Commit-Queue: Martin Šrámek <msramek@chromium.org>
Auto-Submit: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653969}