Use holder dictionary-mode to detect FAST or NORMAL

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/165993004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19518 ce2b1a6d-e550-0410-aec6-3dcde31c8c00