Fix a bug in for/in iteration of arguments objects.

Fix a bug in for/in iteration of arguments objects.

We did not properly combine the property names from the parameter map
and the arguments backing store.  They could overwrite each other and
be unsorted.

Also fix an unrelated bug: deleting from a dictionary-mode arguments
backing store could corrupt the parameter map.

R=rossberg@chromium.org
BUG=1531
TEST=mjsunit/regress/regress-1531.js

Review URL: http://codereview.chromium.org/7278033

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8571 ce2b1a6d-e550-0410-aec6-3dcde31c8c00