Inline Math.fround in optimized code.

R=svenpanne@chromium.org
BUG=v8:3469
LOG=N

Review URL: https://codereview.chromium.org/425943002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22665 ce2b1a6d-e550-0410-aec6-3dcde31c8c00