Symbols for type cells. We can make more efficient code to check against type cells in the future if we use symbols, guaranteed not to conflict with user code. Currently, the "symbols" are the hole and undefined. Undefined may come in from the outside.

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/181283003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19706 ce2b1a6d-e550-0410-aec6-3dcde31c8c00