Update WeakPtr documentation for SequencedTaskRunner support.

Update WeakPtr documentation for SequencedTaskRunner support.

WeakPtr supported SequencedTaskRunner in
https://codereview.chromium.org/20754006, but the
documentation was not updated to match.

BUG=none

Committed: https://crrev.com/e58877edb51b0a0835019246ceb57fd4f2d5cbfc
Cr-Commit-Position: refs/heads/master@{#300717}

[davidben, 2014-06-10 19:02:27]

There was some question as to whether base::WeakPtr supported
SequencedTaskRunners. Apparently the answer is yes now, but the documentation
doesn't match.

[davidben, 2014-10-21 22:36:54]

davidben@chromium.org changed reviewers:
+ mark@chromium.org

[davidben, 2014-10-21 22:36:55]

Apparently I still have this sitting around. +mark since base/OWNERS changed.

[Mark Mentovai, 2014-10-22 13:25:26]

LGTM

[davidben, 2014-10-22 14:32:51]

The CQ bit was checked by davidben@chromium.org

[commit-bot: I haz the power, 2014-10-22 14:33:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/323153002/20001

[commit-bot: I haz the power, 2014-10-22 15:18:00]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-10-22 15:18:01]

Try jobs failed on following builders:
  linux_chromium_asan_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[davidben, 2014-10-22 16:16:10]

The CQ bit was checked by davidben@chromium.org

[commit-bot: I haz the power, 2014-10-22 16:24:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/323153002/20001

[commit-bot: I haz the power, 2014-10-22 16:43:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-10-22 16:43:04]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[davidben, 2014-10-22 16:59:40]

The CQ bit was checked by davidben@chromium.org

[commit-bot: I haz the power, 2014-10-22 17:01:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/323153002/20001

[commit-bot: I haz the power, 2014-10-22 18:37:18]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-10-22 18:38:14]

Patchset 2 (id:??) landed as
https://crrev.com/e58877edb51b0a0835019246ceb57fd4f2d5cbfc
Cr-Commit-Position: refs/heads/master@{#300717}