This is a refactoring only change that switches incremental marking to
use a static object visitor. It also shares the common code between the
non-incremental and the incremental marker. Sharing that would require
semantical changes will be done later.

R=verwaest@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10816007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12193 ce2b1a6d-e550-0410-aec6-3dcde31c8c00