It will record the time-to-schedule-after-job-start for different
task types to try to highlight use cases where contention might
be a problem (and show improvements to it later).

Also introducing AsyncTimedHistogram to support this use case whose
reported timings go beyond a single scope (i.e. the async version of
ScopedTimedHistogram).

Bug: chromium:807606
Change-Id: Ib4d581fa8b001723dfe8c91102280e9608b4fabb
Reviewed-on: https://chromium-review.googlesource.com/899365Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51131}