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

R=yangguo@chromium.org,kbr@chromium.org
BUG=v8:2294

Review URL: https://chromiumcodereview.appspot.com/10831409

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12364 ce2b1a6d-e550-0410-aec6-3dcde31c8c00