This means that ES5 Date Time Strings will default to UTC if timezone is absent.
Handle as many legacy strings as possible the same way as before

BUG=v8:1498
TEST=mjsunit/date

Review URL: http://codereview.chromium.org/7291022

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8513 ce2b1a6d-e550-0410-aec6-3dcde31c8c00