Reuse instance_descriptor field in the map to store additional flags when there are no descriptors. When descriptors get added to the map, move the flags to the DescriptorArray and access through indirection.

Review URL: http://codereview.chromium.org/7033024

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8001 ce2b1a6d-e550-0410-aec6-3dcde31c8c00