Change the representation of catch contexts.

Change the representation of catch contexts.

Before, they had no extra slots and an extension object with one named
property.  Now, they use the extension slot for the property name and have
an extra slot for the thrown object.  This increases the size of the context
itself, but removes overall allocation and eliminates a level of indirection.

R=ager@chromium.org

Review URL: http://codereview.chromium.org/7152002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8277 ce2b1a6d-e550-0410-aec6-3dcde31c8c00