Simplify AST ID generation.

Every CompilationInfo has an ID generator now, and it is never
reset/copied/assigned. Simplified FunctionState.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/633373003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24481 ce2b1a6d-e550-0410-aec6-3dcde31c8c00