MIPS: Make object accessors more const-correct.

Port r21897 (1b152ae)

Fix MIPS implementation of READ_DOUBLE_FIELD() to use const parameter.

Original commit message:
Getting closer to making our IsFOO and FOO::cast methods
const-correct...

BUG=
R=gergely@homejinni.com, jkummerow@chromium.org

Review URL: https://codereview.chromium.org/344203005

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21904 ce2b1a6d-e550-0410-aec6-3dcde31c8c00