Port r12364 (31e40def)

Original commit message:
According to Web IDL spec, we should round to
the nearest integer, choosing the even integer
if it lies halfway between two.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10870049
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12380 ce2b1a6d-e550-0410-aec6-3dcde31c8c00