MIPS: Return Object* instead of MaybeObject* from runtime calls.

Port r20888 (d6883d72)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/246893010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20894 ce2b1a6d-e550-0410-aec6-3dcde31c8c00