This changes the internal convention for marking deleted entries in hash
tables from null_value to the_hole_value, which is consistent with other
usages of the_hole.

R=rossberg@chromium.org,kmillikin@chromium.org
BUG=v8:1622
TEST=mjsunit/harmony/collections

Review URL: http://codereview.chromium.org/8343056

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9871 ce2b1a6d-e550-0410-aec6-3dcde31c8c00