Consistently pass a Heap* to HashTable-related AsObject methods.

This is basically a follow-up CL to https://code.google.com/p/v8/source/detail?r=13908.

BUG=v8:2487

Review URL: https://codereview.chromium.org/12546024

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13911 ce2b1a6d-e550-0410-aec6-3dcde31c8c00