Delegating yield (yield*) should just pass on the iterator results it
receives instead of re-boxing them.

R=rossberg@chromium.org
TEST=mjsunit/harmony/generators-iteration
BUG=

Review URL: https://codereview.chromium.org/16695006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15113 ce2b1a6d-e550-0410-aec6-3dcde31c8c00