Clean up canceled tasks in blink timer unit tests

Currently, 2 of the blink timer unit tests expect a next wake up time
even though they just stopped the timer. That is because
TimeTillNextDelayedTask() counts canceled tasks, which is misleading.

A future feature will cause canceled timer tasks to actually be removed
from the task queue instead of simply being marked as canceled. To
ensure that the test works with and without the feature enabled,
TimeTillNextDelayedTask() now cleans up canceled tasks.

Bug: 1262205
Change-Id: I3fcf96079a91afa3052fff6fe185e45e0d95e196
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3372336


Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#956778}