Always initialize key_type_ in AST nodes

BUG=chromium:423117
LOG=n
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/652183002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24604 ce2b1a6d-e550-0410-aec6-3dcde31c8c00