Call accessors in the global object prototype when initializing global

Call accessors in the global object prototype when initializing global
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