Visit encountered JSWeakCollection list during scavenging.

R=hpayer@chromium.org
BUG=chromium:380068
LOG=N

Review URL: https://codereview.chromium.org/310783003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21655 ce2b1a6d-e550-0410-aec6-3dcde31c8c00