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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

Review URL: https://codereview.chromium.org/1014813002

Cr-Commit-Position: refs/heads/master@{#320915}