MIPS: Turn StringLengthStub (used by string wrappers only) into a hydrogen stub.

Port r19889 (8fa27bb)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/197463009

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19912 ce2b1a6d-e550-0410-aec6-3dcde31c8c00