TaskScheduler [3/9] Task and Sequence
This change is a subset of https://codereview.chromium.org/1698183005/
A Task is a unit of work in the task scheduler. It has a closure,
a sequenced time, TaskTraits and other metadata inherited from
base::PendingTask.
A Sequence holds Tasks that must run in order. It is ref-counted
and has thread-safe Push, Pop and Peek operations. Priority queues,
worker threads and task runners will have references on Sequences.
BUG=553459
Review URL: https://codereview.chromium.org/1705253002
Cr-Commit-Position: refs/heads/master@{#381638}
13 files changed
