Removed a false assertion in ScopeIterator that assumed context extension to never be a JSContextExtensionObject.

The context extension object in a 'with' context is JSContextExtensionObject iff the 'with' statement is generated from a catch block in order to extend its local scope with a variable holding exception object. This is how we differentiate 'catch' scope from 'with' scope.

Chrome bug:
http://code.google.com/p/chromium/issues/detail?id=17229
Review URL: http://codereview.chromium.org/202005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2843 ce2b1a6d-e550-0410-aec6-3dcde31c8c00