Provide Type::Contains methods.

Also, rename all *Currently methods to Now*.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/219523003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20405 ce2b1a6d-e550-0410-aec6-3dcde31c8c00