MIPS: Use mutable heapnumbers to store doubles in fields.

Port r14597 (6e9d7149)

BUG=

Review URL: https://codereview.chromium.org/14607009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14600 ce2b1a6d-e550-0410-aec6-3dcde31c8c00