Unify kMaxArguments with number of bits used to encode it.

Increase the number of bits by 1 by making Flags unsigned.

BUG=chromium:211741

Review URL: https://chromiumcodereview.appspot.com/12886008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13964 ce2b1a6d-e550-0410-aec6-3dcde31c8c00