Refactored ObjectSerializer::VisitFoo functions.

Tiny superficial cleanup to make the similarities more visible.
Removed a few useless assertions on the way.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/60363006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17539 ce2b1a6d-e550-0410-aec6-3dcde31c8c00