Clarify WeakPtr threading characteristic.

Clarify WeakPtr threading characteristic.

Instances must exist, not only the factory, to enforce the thread check.

An example fix this comment is relevant for:
https://codereview.chromium.org/1389553002/

Committed: https://crrev.com/3ad03720b2c3186a681dbc1e0c0bbb6fb8aade24
Cr-Commit-Position: refs/heads/master@{#352445}

[scheib, 2015-10-04 04:35:07]

scheib@chromium.org changed reviewers:
+ thakis@chromium.org

[scheib, 2015-10-04 04:36:53]

scheib@chromium.org changed reviewers:
+ mark@chromium.org
- thakis@chromium.org

[Mark Mentovai, 2015-10-05 15:47:10]

LGTM

[scheib, 2015-10-05 17:09:38]

The CQ bit was checked by scheib@chromium.org

[commit-bot: I haz the power, 2015-10-05 17:11:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1384863002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1384863002/1

[commit-bot: I haz the power, 2015-10-05 22:27:58]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-05 22:28:41]

Patchset 1 (id:??) landed as
https://crrev.com/3ad03720b2c3186a681dbc1e0c0bbb6fb8aade24
Cr-Commit-Position: refs/heads/master@{#352445}