Use the BitField helper class for the code flags, so that we do not have to
define both a shift and a mask explicitly.  This makes changing the flags
layout simpler.

Also, make the 'mask' and 'max' members of BitField into constants, because
they are constant and so that they can be used as constant expressions.
E.g., so they can be used in declaring other const members or in static
asserts.

R=fschneider@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7787028

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9232 ce2b1a6d-e550-0410-aec6-3dcde31c8c00