Revert r24737 "Read object pointer atomically while updating slots"

That broke webkit_unit_tests on ARM64.

BUG=chromium:427746
LOG=Y
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/683113003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24955 ce2b1a6d-e550-0410-aec6-3dcde31c8c00