variables.

Make the declaration of global const variables in the presence of
callbacks a redeclaration error.

Handle const context slot declarations conflicting with a CALLBACK as
a redeclaration error. That is, unless it is on a context extension
object which is not a real object and therefore conceptually have no
accessors in prototype chains. Accessors in prototype chains of
context extension objects are explicitly ignored in SetProperty.

Review URL: http://codereview.chromium.org/6519050

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6841 ce2b1a6d-e550-0410-aec6-3dcde31c8c00