Cleaned up the weak lists hanging off the heap a bit.

 * Route all access to the 3 weak lists through getters/setters.

 * Removed superfluous visiting already done by ProcessWeakReferences.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/273653006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21207 ce2b1a6d-e550-0410-aec6-3dcde31c8c00