Also change a check in heap-inl.h for diagnosing 284577 into an assert.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/24259005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16816 ce2b1a6d-e550-0410-aec6-3dcde31c8c00