Turn ToNumberStub into a PlatformCodeStub again.

The HydrogenCodeStub is too expensive and there's no easy way to reduce
this cost, so turning it into a PlatformCodeStub solves that problem
until we can use TurboFan for code stubs.

TEST=mjsunit
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/683913008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25187 ce2b1a6d-e550-0410-aec6-3dcde31c8c00