[base/task] Create skeleton for base/task.

Create base/task/task_scheduler and base/task/sequence_manager
directories, add OWNERS files and README.md stub.

R=gab@chromium.org
BUG=783309

Change-Id: Ic30cfac8ce71e5f0553fcfb6c95e05750d15d265
Reviewed-on: https://chromium-review.googlesource.com/934444
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539455}

base/task/OWNERS

+fdoray@chromium.org
+gab@chromium.org
+robliao@chromium.org
+
+# TEAM: scheduler-dev@chromium.org
+# COMPONENT: Internals>TaskScheduler

base/task/README.md

+This directory has the following layout (WIP):
+- base/task/: public APIs for posting tasks and managing task queues.
+- base/task/task_scheduler/: implementation of the TaskScheduler.
+- base/task/sequence_manager/: implementation of the SequenceManager.
+
+Apart from embedders explicitly managing a TaskScheduler and/or SequenceManager
+instance(s) for their process/threads, the vast majority of users should only
+need APIs in base/task/.
+
+Documentation:
+- [Threading and tasks](https://chromium.googlesource.com/chromium/src/+/lkcr/docs/threading_and_tasks.md)
+- [Callbacks](https://chromium.googlesource.com/chromium/src/+/lkcr/docs/callback.md)

base/task/sequence_manager/OWNERS

+altimin@chromium.org
+alexclarke@chromium.org
+skyostil@chromium.org
+
+# TEAM: scheduler-dev@chromium.org
+# Component: Blink>Scheduling

base/task/task_scheduler/OWNERS

+fdoray@chromium.org
+gab@chromium.org
+robliao@chromium.org
+
+# TEAM: scheduler-dev@chromium.org
+# COMPONENT: Internals>TaskScheduler