MIPS: make isolate accessible from returnvalue

Port r14819 (1ee1db14)

BUG=

Review URL: https://codereview.chromium.org/16099003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14827 ce2b1a6d-e550-0410-aec6-3dcde31c8c00