TaskScheduler: Minor clarifications to SchedulerLock code

This is a follow-up to https://chromium-review.googlesource.com/c/chromium/src/+/1320650

Bug: 889029
Change-Id: I4dd60a56fcab509578cdfe08c999c1e34fd72375
Reviewed-on: https://chromium-review.googlesource.com/c/1331219
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607257}
3 files changed