Do not visit smis in the root list during GC.

R=mstarzinger@chromium.org
BUG=328804
LOG=N

Review URL: https://codereview.chromium.org/166023003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19400 ce2b1a6d-e550-0410-aec6-3dcde31c8c00