Add support for storage type to base::Flags.

Also drop the DEFINE_FLAGS() macro, and use the typedef explicitly.

TEST=base-unittests
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/527173002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23578 ce2b1a6d-e550-0410-aec6-3dcde31c8c00