Port threading design doc to in-tree docs, start a README for design docs in the tree.

Port threading design doc to in-tree docs, start a README for design docs in the tree. 

This is a straight up port of https://www.chromium.org/developers/design-documents/threading, with minimal editing only for style consistency and some grammar errors.

Review-Url: https://codereview.chromium.org/2822353002
Cr-Commit-Position: refs/heads/master@{#466049}
Committed: https://chromium.googlesource.com/chromium/src/+/ad589ceb60717d4314214001cf69a9d32cd2f193

[Carlson, 2017-04-19 00:10:00]

Description was changed from

==========
Port threading design doc to in-tree docs, start a README for design docs in the
tree.
==========

to

==========
Port threading design doc to in-tree docs, start a README for design docs in the
tree. 

This is a straight up port of
https://www.chromium.org/developers/design-documents/threading, with minimal
editing only for style consistency and some grammar errors.
==========

[Carlson, 2017-04-19 00:10:11]

justincarlson@chromium.org changed reviewers:
+ dpranke@chromium.org

[Dirk Pranke, 2017-04-19 00:22:03]

rs- lgtm.

[Carlson, 2017-04-20 16:50:41]

The CQ bit was checked by justincarlson@chromium.org

[commit-bot: I haz the power, 2017-04-20 16:51:33]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-20 17:26:11]

CQ is committing da patch.

Bot data: {"patchset_id": 40001, "attempt_start_ts": 1492707041132230,
"parent_rev": "322f0d1c2543907fcc64ebcf55845d91c8b7e88b", "commit_rev":
"ad589ceb60717d4314214001cf69a9d32cd2f193"}

[commit-bot: I haz the power, 2017-04-20 17:26:17]

Description was changed from

==========
Port threading design doc to in-tree docs, start a README for design docs in the
tree. 

This is a straight up port of
https://www.chromium.org/developers/design-documents/threading, with minimal
editing only for style consistency and some grammar errors.
==========

to

==========
Port threading design doc to in-tree docs, start a README for design docs in the
tree. 

This is a straight up port of
https://www.chromium.org/developers/design-documents/threading, with minimal
editing only for style consistency and some grammar errors.

Review-Url: https://codereview.chromium.org/2822353002
Cr-Commit-Position: refs/heads/master@{#466049}
Committed:
https://chromium.googlesource.com/chromium/src/+/ad589ceb60717d4314214001cf69...
==========

[commit-bot: I haz the power, 2017-04-20 17:26:18]

Committed patchset #3 (id:40001) as
https://chromium.googlesource.com/chromium/src/+/ad589ceb60717d4314214001cf69...