MIPS: Minor-key-ify remaining code stubs.

Port r23643 (0051736b)

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/536193002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23668 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: CallDescriptors::InitializeForIsolate() is no longer needed.

Port r23652 (3bd900e4)

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/537753002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23669 ce2b1a6d-e550-0410-aec6-3dcde31c8c00