Move logic for hidden properties into the JSObject.

Move logic for hidden properties into the JSObject.

Previously, the logic using the hidden properties backing object was
spread accross use sites. Now it's all contained in JSObject, with
only simple accessors available.
Also change the backing object to be a StringDictionary rather than a JSObject.
There's still room for improvement by making a hash-table that don't
store property details as well.

Review URL: http://codereview.chromium.org/8050013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9510 ce2b1a6d-e550-0410-aec6-3dcde31c8c00