In-object double fields unboxing (for 64-bit only).

This CL introduces LayoutDescriptor which is responsible for tracking which in-object fields are tagged and which are not.
LayoutDescriptor field added to Map. Currently unboxing is disabled.

R=hpayer@chromium.org, verwaest@chromium.org

Review URL: https://codereview.chromium.org/391693002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25250 ce2b1a6d-e550-0410-aec6-3dcde31c8c00