Merged r18783, r18784 into trunk branch.

MIPS: Reland r18714 'Unify calling to GenerateFastApiCallBody before stubbing it'.

MIPS: Turn FastNewContextStub into a HydrogenCodeStub.

R=jkummerow@chromium.org, machenbach@chromium.org
BUG=

Review URL: https://codereview.chromium.org/145853002

git-svn-id: https://v8.googlecode.com/svn/trunk@18793 ce2b1a6d-e550-0410-aec6-3dcde31c8c00