Replace C++ bitfields with our own BitFields

Shave this yak from orbit, it's the only way to be sure.

BUG=chromium:427616
LOG=n
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/700963002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25148 ce2b1a6d-e550-0410-aec6-3dcde31c8c00