Allow variable proxies for the same global variable to share the same

Allow variable proxies for the same global variable to share the same
variable object.

Add a map from names to variables for global scopes just like
non-global scopes.  Variables are added to the map by the parser when
it encounters a declaration in a global scope or else at scope
resolution time by a failed variable lookup from the global scope or
an inner one and with no intervening with statements or possible calls
to eval.
Review URL: http://codereview.chromium.org/149245

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2369 ce2b1a6d-e550-0410-aec6-3dcde31c8c00