Make sure when we shrink an object that we store a filler first into the free memory before updating the map or size.

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/219103002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20361 ce2b1a6d-e550-0410-aec6-3dcde31c8c00