[iOS] WeakNSObject assignment is consistent with copy constructor

[iOS] WeakNSObject assignment is consistent with copy constructor

The copy constructor was changed (in CL
https://codereview.chromium.org/853503002/) to allow a WeakNSObject to
be copied on a different thread.
This CL is making the assignment operator behave consistently with the
copy constructor: it is now allowed to assign a WeakNSObject on a
different thread.

In any case, dereferencing the WeakNSObject on another thread remains
disallowed.

BUG=394008
Committed: https://crrev.com/1fd547a90acc75c995d8ec901ba52879558a27ae
Cr-Commit-Position: refs/heads/master@{#320915}

[droger, 2015-03-17 12:50:32]

droger@chromium.org changed reviewers:
+ stuartmorgan@chromium.org

[stuartmorgan, 2015-03-17 14:48:11]

lgtm

[droger, 2015-03-17 15:09:07]

Thanks for the review.

[droger, 2015-03-17 15:09:10]

The CQ bit was checked by droger@chromium.org

[commit-bot: I haz the power, 2015-03-17 15:09:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1014813002/1

[commit-bot: I haz the power, 2015-03-17 15:13:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-17 15:14:09]

Patchset 1 (id:??) landed as
https://crrev.com/1fd547a90acc75c995d8ec901ba52879558a27ae
Cr-Commit-Position: refs/heads/master@{#320915}