Revert "Enable concurrent sweeping."

[Sheriff intervening.]

TBR=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/254003002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21061 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
disable Object.observe

R=danno@chromium.org, danno
BUG=

Review URL: https://codereview.chromium.org/252063003

Patch from Rafael Weinstein <rafaelw@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21062 ce2b1a6d-e550-0410-aec6-3dcde31c8c00