[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: http://src.chromium.org/svn/trunk/src@48857 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: ad8eade1515c75929cd0603d2185d1742016ab7b
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser