Make expressions like "if (weak_ptr)" work

Make expressions like "if (weak_ptr)" work

Use the same trick as scoped_ptr<T>.

Add comments regarding == and != operators and their relation to
the bool trick.

BUG=245942
R=darin@chromium.org, rsleevi@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=204438

[akalin, 2013-06-04 19:58:34]

+rsleevi for review

+darin for owners (will probably TBR)

[darin (slow to review), 2013-06-04 20:10:20]

LGTM

[akalin, 2013-06-04 21:02:34]

+enne FYI

[enne (OOO), 2013-06-04 21:05:10]

On 2013/06/04 21:02:34, akalin wrote:
> +enne FYI

Thanks for the extra comments.  :)

[Ryan Sleevi, 2013-06-04 21:10:35]

lgtm

[commit-bot: I haz the power, 2013-06-05 18:22:42]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/akalin@chromium.org/15721007/17001

[commit-bot: I haz the power, 2013-06-06 01:29:25]

Retried try job too often on win_rel for step(s) chrome_frame_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2013-06-06 03:06:10]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/akalin@chromium.org/15721007/17001

[commit-bot: I haz the power, 2013-06-06 03:59:59]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/akalin@chromium.org/15721007/51001

[akalin, 2013-06-06 05:32:52]

Committed patchset #3 manually as r204438 (presubmit successful).