[Mac] Tighten up objc zombie dealloc implementation.
[Mac] Tighten up objc zombie dealloc implementation.

Enabling the zombie code in release causes Perf(2) to regress.
Reviewing |ZombieDealloc()|, I notice that object_setClass() is
implemented with a memory barrier.  The objc runtime doesn't use it in
deallocation, so move to direct access.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/2605004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@48857 0039d316-1c4b-4281-b951-d872f2087c98

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