Turn assert into RUNTIME_ASSERT

TBR=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/350683003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21926 ce2b1a6d-e550-0410-aec6-3dcde31c8c00