runtime: convert timeHistogram to atomic types

runtime: convert timeHistogram to atomic types

I've dropped the note that sched.timeToRun is protected by sched.lock,
as it does not seem to be true.

For #53821.

Change-Id: I03f8dc6ca0bcd4ccf3ec113010a0aa39c6f7d6ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/419449
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>