Reland "Fix usage of PostCancelableDelayedTask in PSIMemoryMetrics"

This is a reland of 37f856365d7f22fa137b5b99765b339501150f53

The issue was using GUARDED_BY() instead of GUARDED_BY_CONTEXT().

Original change's description:
> Fix usage of PostCancelableDelayedTask in PSIMemoryMetrics
>
> PostCancelableDelayedTask() must be called from the sequence where
> the task is posted. Currently, it can either be called from Start(),
> which runs on the main thread, or as a continuation in
> CollectEventsAndReschedule(), which runs on the background sequence.
>
> To fix this, Start() now posts ScheduleCollector() to the background
> sequence. A sequence checker was added to ensure that |last_timer_| is
> always used on the right sequence.
>
> Change-Id: Ibc679bd25b785692ad831ddc5bfdb9578d279431
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3400119
> Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
> Commit-Queue: Patrick Monette <pmonette@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#961818}

Change-Id: I8d25e7f1637f739ce506d6f5b06d3e5625600068
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3404913
Auto-Submit: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962841}