MIPS: Serializer enable/disable flags need thread safety.

Port r20855 (ac70e129)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/239173004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20859 ce2b1a6d-e550-0410-aec6-3dcde31c8c00