Fix issue with context not being saved on x64 introduced in 144543004

TBR=verwaest@chromium.org

BUG=

Review URL: https://codereview.chromium.org/143333003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18845 ce2b1a6d-e550-0410-aec6-3dcde31c8c00